diff --git a/README.md b/README.md
index d60c856..1eff7ed 100644
--- a/README.md
+++ b/README.md
@@ -237,6 +237,23 @@ interface type and the second argument is a zero value of the concrete type. Any
 set that includes an interface binding must also have a provider in the same set
 that provides the concrete type.
 
+If necessary, you can also bind one interface to another:
+
+```go
+type FooerPlus interface {
+  Fooer
+  Bar() String
+}
+
+func ProviderFooerPlus() FooerPlus {
+  ...
+}
+
+var FooerPlusAsFooer = wire.NewSet(
+  ProvideFooerPlus,
+  wire.Bind(new(Fooer), *new(FooerPlus)))
+```
+
 [type identity]: https://golang.org/ref/spec#Type_identity
 [return concrete types]: https://github.com/golang/go/wiki/CodeReviewComments#interfaces
 
diff --git a/internal/wire/parse.go b/internal/wire/parse.go
index 1b65312..2450100 100644
--- a/internal/wire/parse.go
+++ b/internal/wire/parse.go
@@ -680,22 +680,23 @@ func processBind(fset *token.FileSet, info *types.Info, call *ast.CallExpr) (*If
 	ifaceArgType := info.TypeOf(call.Args[0])
 	ifacePtr, ok := ifaceArgType.(*types.Pointer)
 	if !ok {
-		return nil, notePosition(fset.Position(call.Pos()), fmt.Errorf("first argument to bind must be a pointer to an interface type; found %s", types.TypeString(ifaceArgType, nil)))
+		return nil, notePosition(fset.Position(call.Pos()), fmt.Errorf("first argument to Bind must be a pointer to an interface type; found %s", types.TypeString(ifaceArgType, nil)))
 	}
-	methodSet, ok := ifacePtr.Elem().Underlying().(*types.Interface)
+	iface := ifacePtr.Elem()
+	methodSet, ok := iface.Underlying().(*types.Interface)
 	if !ok {
-		return nil, notePosition(fset.Position(call.Pos()), fmt.Errorf("first argument to bind must be a pointer to an interface type; found %s", types.TypeString(ifaceArgType, nil)))
+		return nil, notePosition(fset.Position(call.Pos()), fmt.Errorf("first argument to Bind must be a pointer to an interface type; found %s", types.TypeString(ifaceArgType, nil)))
 	}
 	provided := info.TypeOf(call.Args[1])
-	if types.Identical(ifacePtr.Elem(), provided) {
+	if types.Identical(iface, provided) {
 		return nil, notePosition(fset.Position(call.Pos()), errors.New("cannot bind interface to itself"))
 	}
 	if !types.Implements(provided, methodSet) {
-		return nil, notePosition(fset.Position(call.Pos()), fmt.Errorf("%s does not implement %s", types.TypeString(provided, nil), types.TypeString(ifaceArgType, nil)))
+		return nil, notePosition(fset.Position(call.Pos()), fmt.Errorf("%s does not implement %s", types.TypeString(provided, nil), types.TypeString(iface, nil)))
 	}
 	return &IfaceBinding{
 		Pos:      call.Pos(),
-		Iface:    ifacePtr.Elem(),
+		Iface:    iface,
 		Provided: provided,
 	}, nil
 }
@@ -764,7 +765,7 @@ func processInterfaceValue(fset *token.FileSet, info *types.Info, call *ast.Call
 	}
 	provided := info.TypeOf(call.Args[1])
 	if !types.Implements(provided, methodSet) {
-		return nil, notePosition(fset.Position(call.Pos()), fmt.Errorf("%s does not implement %s", types.TypeString(provided, nil), types.TypeString(ifaceArgType, nil)))
+		return nil, notePosition(fset.Position(call.Pos()), fmt.Errorf("%s does not implement %s", types.TypeString(provided, nil), types.TypeString(iface, nil)))
 	}
 	return &Value{
 		Pos:  call.Args[1].Pos(),
diff --git a/internal/wire/testdata/InterfaceBindingDoesntImplement/foo/foo.go b/internal/wire/testdata/InterfaceBindingDoesntImplement/foo/foo.go
new file mode 100644
index 0000000..4942cb8
--- /dev/null
+++ b/internal/wire/testdata/InterfaceBindingDoesntImplement/foo/foo.go
@@ -0,0 +1,27 @@
+// Copyright 2018 The Go Cloud Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package main
+
+import (
+	"fmt"
+)
+
+func main() {
+	fmt.Println(injectFooer().Foo())
+}
+
+type Fooer interface {
+	Foo() string
+}
diff --git a/internal/wire/testdata/InterfaceBindingDoesntImplement/foo/wire.go b/internal/wire/testdata/InterfaceBindingDoesntImplement/foo/wire.go
new file mode 100644
index 0000000..454f839
--- /dev/null
+++ b/internal/wire/testdata/InterfaceBindingDoesntImplement/foo/wire.go
@@ -0,0 +1,27 @@
+// Copyright 2018 The Go Cloud Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//+build wireinject
+
+package main
+
+import (
+	"github.com/google/go-cloud/wire"
+)
+
+func injectFooer() Fooer {
+	// wrong: string doesn't implement Fooer.
+	wire.Build(wire.Bind((*Fooer)(nil), "foo"))
+	return nil
+}
diff --git a/internal/wire/testdata/InterfaceBindingDoesntImplement/pkg b/internal/wire/testdata/InterfaceBindingDoesntImplement/pkg
new file mode 100644
index 0000000..f7a5c8c
--- /dev/null
+++ b/internal/wire/testdata/InterfaceBindingDoesntImplement/pkg
@@ -0,0 +1 @@
+example.com/foo
diff --git a/internal/wire/testdata/InterfaceBindingDoesntImplement/want/wire_errs.txt b/internal/wire/testdata/InterfaceBindingDoesntImplement/want/wire_errs.txt
new file mode 100644
index 0000000..c92114a
--- /dev/null
+++ b/internal/wire/testdata/InterfaceBindingDoesntImplement/want/wire_errs.txt
@@ -0,0 +1 @@
+string does not implement example.com/foo.Fooer
diff --git a/internal/wire/testdata/InterfaceBindingInvalidArg0/foo/foo.go b/internal/wire/testdata/InterfaceBindingInvalidArg0/foo/foo.go
new file mode 100644
index 0000000..4942cb8
--- /dev/null
+++ b/internal/wire/testdata/InterfaceBindingInvalidArg0/foo/foo.go
@@ -0,0 +1,27 @@
+// Copyright 2018 The Go Cloud Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package main
+
+import (
+	"fmt"
+)
+
+func main() {
+	fmt.Println(injectFooer().Foo())
+}
+
+type Fooer interface {
+	Foo() string
+}
diff --git a/internal/wire/testdata/InterfaceBindingInvalidArg0/foo/wire.go b/internal/wire/testdata/InterfaceBindingInvalidArg0/foo/wire.go
new file mode 100644
index 0000000..6a504e5
--- /dev/null
+++ b/internal/wire/testdata/InterfaceBindingInvalidArg0/foo/wire.go
@@ -0,0 +1,27 @@
+// Copyright 2018 The Go Cloud Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//+build wireinject
+
+package main
+
+import (
+	"github.com/google/go-cloud/wire"
+)
+
+func injectFooer() Fooer {
+	// wrong: arg0 must be a pointer to an interface.
+	wire.Build(wire.Bind("foo", "bar"))
+	return nil
+}
diff --git a/internal/wire/testdata/InterfaceBindingInvalidArg0/pkg b/internal/wire/testdata/InterfaceBindingInvalidArg0/pkg
new file mode 100644
index 0000000..f7a5c8c
--- /dev/null
+++ b/internal/wire/testdata/InterfaceBindingInvalidArg0/pkg
@@ -0,0 +1 @@
+example.com/foo
diff --git a/internal/wire/testdata/InterfaceBindingInvalidArg0/want/wire_errs.txt b/internal/wire/testdata/InterfaceBindingInvalidArg0/want/wire_errs.txt
new file mode 100644
index 0000000..6c4aa11
--- /dev/null
+++ b/internal/wire/testdata/InterfaceBindingInvalidArg0/want/wire_errs.txt
@@ -0,0 +1 @@
+first argument to Bind must be a pointer to an interface type; found string
diff --git a/internal/wire/testdata/InterfaceBindingNotEnoughArgs/foo/foo.go b/internal/wire/testdata/InterfaceBindingNotEnoughArgs/foo/foo.go
new file mode 100644
index 0000000..4942cb8
--- /dev/null
+++ b/internal/wire/testdata/InterfaceBindingNotEnoughArgs/foo/foo.go
@@ -0,0 +1,27 @@
+// Copyright 2018 The Go Cloud Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package main
+
+import (
+	"fmt"
+)
+
+func main() {
+	fmt.Println(injectFooer().Foo())
+}
+
+type Fooer interface {
+	Foo() string
+}
diff --git a/internal/wire/testdata/InterfaceBindingNotEnoughArgs/foo/wire.go b/internal/wire/testdata/InterfaceBindingNotEnoughArgs/foo/wire.go
new file mode 100644
index 0000000..b443d9a
--- /dev/null
+++ b/internal/wire/testdata/InterfaceBindingNotEnoughArgs/foo/wire.go
@@ -0,0 +1,27 @@
+// Copyright 2018 The Go Cloud Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//+build wireinject
+
+package main
+
+import (
+	"github.com/google/go-cloud/wire"
+)
+
+func injectFooer() Fooer {
+	// wrong: wire.Bind requires 2 args.
+	wire.Build(wire.Bind((*Fooer)(nil)))
+	return nil
+}
diff --git a/internal/wire/testdata/InterfaceBindingNotEnoughArgs/pkg b/internal/wire/testdata/InterfaceBindingNotEnoughArgs/pkg
new file mode 100644
index 0000000..f7a5c8c
--- /dev/null
+++ b/internal/wire/testdata/InterfaceBindingNotEnoughArgs/pkg
@@ -0,0 +1 @@
+example.com/foo
diff --git a/internal/wire/testdata/InterfaceBindingNotEnoughArgs/want/wire_errs.txt b/internal/wire/testdata/InterfaceBindingNotEnoughArgs/want/wire_errs.txt
new file mode 100644
index 0000000..ffd395a
--- /dev/null
+++ b/internal/wire/testdata/InterfaceBindingNotEnoughArgs/want/wire_errs.txt
@@ -0,0 +1 @@
+too few arguments in call to wire.Bind
diff --git a/internal/wire/testdata/InterfaceValueDoesntImplement/foo/foo.go b/internal/wire/testdata/InterfaceValueDoesntImplement/foo/foo.go
new file mode 100644
index 0000000..4289d9f
--- /dev/null
+++ b/internal/wire/testdata/InterfaceValueDoesntImplement/foo/foo.go
@@ -0,0 +1,21 @@
+// Copyright 2018 The Go Cloud Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package main
+
+import "fmt"
+
+func main() {
+	fmt.Println(injectedMessage())
+}
diff --git a/internal/wire/testdata/InterfaceValueDoesntImplement/foo/wire.go b/internal/wire/testdata/InterfaceValueDoesntImplement/foo/wire.go
new file mode 100644
index 0000000..58d28e1
--- /dev/null
+++ b/internal/wire/testdata/InterfaceValueDoesntImplement/foo/wire.go
@@ -0,0 +1,28 @@
+// Copyright 2018 The Go Cloud Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//+build wireinject
+
+package main
+
+import (
+	"github.com/google/go-cloud/wire"
+	"io"
+)
+
+func injectedMessage() string {
+	// wrong: string doesn't implement io.Reader.
+	wire.Build(wire.InterfaceValue(new(io.Reader), "bar"))
+	return ""
+}
diff --git a/internal/wire/testdata/InterfaceValueDoesntImplement/pkg b/internal/wire/testdata/InterfaceValueDoesntImplement/pkg
new file mode 100644
index 0000000..f7a5c8c
--- /dev/null
+++ b/internal/wire/testdata/InterfaceValueDoesntImplement/pkg
@@ -0,0 +1 @@
+example.com/foo
diff --git a/internal/wire/testdata/InterfaceValueDoesntImplement/want/wire_errs.txt b/internal/wire/testdata/InterfaceValueDoesntImplement/want/wire_errs.txt
new file mode 100644
index 0000000..d88c9ce
--- /dev/null
+++ b/internal/wire/testdata/InterfaceValueDoesntImplement/want/wire_errs.txt
@@ -0,0 +1 @@
+string does not implement io.Reader
diff --git a/internal/wire/testdata/InterfaceValueInvalidArg0/foo/wire.go b/internal/wire/testdata/InterfaceValueInvalidArg0/foo/wire.go
index fa64bec..5906a6d 100644
--- a/internal/wire/testdata/InterfaceValueInvalidArg0/foo/wire.go
+++ b/internal/wire/testdata/InterfaceValueInvalidArg0/foo/wire.go
@@ -21,6 +21,7 @@ import (
 )
 
 func injectedMessage() string {
+	// wrong: arg0 must be a pointer to an interface.
 	wire.Build(wire.InterfaceValue("foo", "bar"))
 	return ""
 }
diff --git a/internal/wire/testdata/InterfaceValueNotEnoughArgs/foo/wire.go b/internal/wire/testdata/InterfaceValueNotEnoughArgs/foo/wire.go
index 17576ae..718322f 100644
--- a/internal/wire/testdata/InterfaceValueNotEnoughArgs/foo/wire.go
+++ b/internal/wire/testdata/InterfaceValueNotEnoughArgs/foo/wire.go
@@ -21,6 +21,7 @@ import (
 )
 
 func injectedMessage() string {
+	// wrong: InterfaceValue requires 2 args.
 	wire.Build(wire.InterfaceValue("foo"))
 	return ""
 }