Use the zero value when decoding a map element that is missing its key or value. Signed-off-by: David Symonds <dsymonds@golang.org>

diff --git a/proto/all_test.go b/proto/all_test.go
index f68f913..4888308 100644
--- a/proto/all_test.go
+++ b/proto/all_test.go

@@ -1958,6 +1958,40 @@
 	}
 }
 
+func TestDecodeMapFieldMissingKey(t *testing.T) {
+	b := []byte{
+		0x0A, 0x03, // message, tag 1 (name_mapping), of length 3 bytes
+		// no key
+		0x12, 0x01, 0x6D, // string value of length 1 byte, value "m"
+	}
+	got := &MessageWithMap{}
+	err := Unmarshal(b, got)
+	if err != nil {
+		t.Fatalf("failed to marshal map with missing key: %v", err)
+	}
+	want := &MessageWithMap{NameMapping: map[int32]string{0: "m"}}
+	if !Equal(got, want) {
+		t.Errorf("Unmarshaled map with no key was not as expected. got: %v, want %v", got, want)
+	}
+}
+
+func TestDecodeMapFieldMissingValue(t *testing.T) {
+	b := []byte{
+		0x0A, 0x02, // message, tag 1 (name_mapping), of length 2 bytes
+		0x08, 0x01, // varint key, value 1
+		// no value
+	}
+	got := &MessageWithMap{}
+	err := Unmarshal(b, got)
+	if err != nil {
+		t.Fatalf("failed to marshal map with missing value: %v", err)
+	}
+	want := &MessageWithMap{NameMapping: map[int32]string{1: ""}}
+	if !Equal(got, want) {
+		t.Errorf("Unmarshaled map with no value was not as expected. got: %v, want %v", got, want)
+	}
+}
+
 func TestOneof(t *testing.T) {
 	m := &Communique{}
 	b, err := Marshal(m)

diff --git a/proto/decode.go b/proto/decode.go
index 5810782..f94b9f4 100644
--- a/proto/decode.go
+++ b/proto/decode.go

@@ -768,10 +768,11 @@
 		}
 	}
 	keyelem, valelem := keyptr.Elem(), valptr.Elem()
-	if !keyelem.IsValid() || !valelem.IsValid() {
-		// We did not decode the key or the value in the map entry.
-		// Either way, it's an invalid map entry.
-		return fmt.Errorf("proto: bad map data: missing key/val")
+	if !keyelem.IsValid() {
+		keyelem = reflect.Zero(p.mtype.Key())
+	}
+	if !valelem.IsValid() {
+		valelem = reflect.Zero(p.mtype.Elem())
 	}
 
 	v.SetMapIndex(keyelem, valelem)