diff --git a/cmd/protoc-gen-go/internal_gengo/well_known_types.go b/cmd/protoc-gen-go/internal_gengo/well_known_types.go
index 8b12823..7f6ca2c 100644
--- a/cmd/protoc-gen-go/internal_gengo/well_known_types.go
+++ b/cmd/protoc-gen-go/internal_gengo/well_known_types.go
@@ -54,13 +54,13 @@
 		g.P("//")
 		g.P("// If no options are specified, call src.UnmarshalTo instead.")
 		g.P("func UnmarshalTo(src *Any, dst ", protoPackage.Ident("Message"), ", opts ", protoPackage.Ident("UnmarshalOptions"), ") error {")
-		g.P("	if src.GetTypeUrl() == \"\" {")
-		g.P("		return ", protoimplPackage.Ident("X"), ".NewError(\"invalid empty type URL\")")
+		g.P("	if src == nil {")
+		g.P("		return ", protoimplPackage.Ident("X"), ".NewError(\"invalid nil source message\")")
 		g.P("	}")
 		g.P("	if !src.MessageIs(dst) {")
 		g.P("		got := dst.ProtoReflect().Descriptor().FullName()")
 		g.P("		want := src.MessageName()")
-		g.P("		return ", protoimplPackage.Ident("X"), ".NewError(\"mismatching message types: got %q, want %q\", got, want)")
+		g.P("		return ", protoimplPackage.Ident("X"), ".NewError(\"mismatched message type: got %q, want %q\", got, want)")
 		g.P("	}")
 		g.P("	return opts.Unmarshal(src.GetValue(), dst)")
 		g.P("}")
@@ -608,7 +608,7 @@
 		g.P("// It reports false for a nil FieldMask.")
 		g.P("func (x *FieldMask) IsValid(m ", protoPackage.Ident("Message"), ") bool {")
 		g.P("	paths := x.GetPaths()")
-		g.P("	return numValidPaths(m, paths) == len(paths)")
+		g.P("	return x != nil && numValidPaths(m, paths) == len(paths)")
 		g.P("}")
 		g.P()
 
diff --git a/types/known/anypb/any.pb.go b/types/known/anypb/any.pb.go
index 061a9ee..47bc555 100644
--- a/types/known/anypb/any.pb.go
+++ b/types/known/anypb/any.pb.go
@@ -194,13 +194,13 @@
 //
 // If no options are specified, call src.UnmarshalTo instead.
 func UnmarshalTo(src *Any, dst proto.Message, opts proto.UnmarshalOptions) error {
-	if src.GetTypeUrl() == "" {
-		return protoimpl.X.NewError("invalid empty type URL")
+	if src == nil {
+		return protoimpl.X.NewError("invalid nil source message")
 	}
 	if !src.MessageIs(dst) {
 		got := dst.ProtoReflect().Descriptor().FullName()
 		want := src.MessageName()
-		return protoimpl.X.NewError("mismatching message types: got %q, want %q", got, want)
+		return protoimpl.X.NewError("mismatched message type: got %q, want %q", got, want)
 	}
 	return opts.Unmarshal(src.GetValue(), dst)
 }
diff --git a/types/known/fieldmaskpb/field_mask.pb.go b/types/known/fieldmaskpb/field_mask.pb.go
index ae1be0e..da32aa5 100644
--- a/types/known/fieldmaskpb/field_mask.pb.go
+++ b/types/known/fieldmaskpb/field_mask.pb.go
@@ -307,7 +307,7 @@
 // It reports false for a nil FieldMask.
 func (x *FieldMask) IsValid(m proto.Message) bool {
 	paths := x.GetPaths()
-	return numValidPaths(m, paths) == len(paths)
+	return x != nil && numValidPaths(m, paths) == len(paths)
 }
 
 // Append appends a list of paths to the mask and verifies that each one