diff --git a/internal/impl/message_reflect.go b/internal/impl/message_reflect.go
index ecb4623d..98ab94ae 100644
--- a/internal/impl/message_reflect.go
+++ b/internal/impl/message_reflect.go
@@ -85,7 +85,9 @@ func (mi *MessageInfo) makeKnownFieldsFunc(si structInfo) {
 	mi.oneofs = map[protoreflect.Name]*oneofInfo{}
 	for i := 0; i < md.Oneofs().Len(); i++ {
 		od := md.Oneofs().Get(i)
-		mi.oneofs[od.Name()] = makeOneofInfo(od, si, mi.Exporter)
+		if !od.IsSynthetic() {
+			mi.oneofs[od.Name()] = makeOneofInfo(od, si, mi.Exporter)
+		}
 	}
 
 	mi.denseFields = make([]*fieldInfo, fds.Len()*2)
diff --git a/testing/prototest/message.go b/testing/prototest/message.go
index ebe1b871..eaf53cfe 100644
--- a/testing/prototest/message.go
+++ b/testing/prototest/message.go
@@ -579,8 +579,13 @@ func testOneof(t testing.TB, m protoreflect.Message, od protoreflect.OneofDescri
 				// Set fields explicitly.
 				m.Set(fda, newValue(m, fda, 1, nil))
 			}
-			if got, want := m.WhichOneof(od), fda; got != want {
-				t.Errorf("after setting oneof field %q:\nWhichOneof(%q) = %v, want %v", fda.FullName(), fda.Name(), got, want)
+			if !od.IsSynthetic() {
+				// Synthetic oneofs are used to represent optional fields in
+				// proto3. While they show up in protoreflect, WhichOneof does
+				// not work on these (only on non-synthetic, explicit oneofs).
+				if got, want := m.WhichOneof(od), fda; got != want {
+					t.Errorf("after setting oneof field %q:\nWhichOneof(%q) = %v, want %v", fda.FullName(), fda.Name(), got, want)
+				}
 			}
 			for j := 0; j < od.Fields().Len(); j++ {
 				fdb := od.Fields().Get(j)