diff --git a/proto/extensions.go b/proto/extensions.go
index d02cadc..9f840fa 100644
--- a/proto/extensions.go
+++ b/proto/extensions.go
@@ -41,6 +41,9 @@
 	"strconv"
 )
 
+// ErrMissingExtension is the error returned by GetExtension if the named extension is not in the message.
+var ErrMissingExtension = errors.New("proto: missing extension")
+
 // ExtensionRange represents a range of message extensions for a protocol buffer.
 // Used in code generated by the protocol compiler.
 type ExtensionRange struct {
@@ -153,7 +156,7 @@
 }
 
 // GetExtension parses and returns the given extension of pb.
-// If the extension is not present it returns (nil, nil).
+// If the extension is not present it returns ErrMissingExtension.
 func GetExtension(pb extendableProto, extension *ExtensionDesc) (interface{}, error) {
 	if err := checkExtensionTypes(pb, extension); err != nil {
 		return nil, err
@@ -161,7 +164,7 @@
 
 	e, ok := pb.ExtensionMap()[extension.Field]
 	if !ok {
-		return nil, nil // not an error
+		return nil, ErrMissingExtension
 	}
 	if e.value != nil {
 		// Already decoded. Check the descriptor, though.