diff --git a/integ/rpc_test.go b/integ/rpc_test.go
index ba52189..314f2ed 100644
--- a/integ/rpc_test.go
+++ b/integ/rpc_test.go
@@ -30,6 +30,11 @@ func TestRPCWorkflow(t *testing.T) {
 	rpcErr, _ := err.(*iwf.ApiError)
 	assert.Equal(t, "worker API error, status:501, errorType:test-error-type", rpcErr.Response.GetDetail())
 
+	// Test unregister client
+	unregClient := iwf.NewUnregisteredClient(nil)
+	err = unregClient.InvokeRPCByName(context.Background(), wfId, "", "TestErrorRPC", 1, nil, nil)
+	assert.NotNil(t, err)
+
 	var rpcOutput int
 	err = client.InvokeRPC(context.Background(), wfId, "", wf.TestRPC, 1, &rpcOutput)
 	assert.Nil(t, err)
diff --git a/iwf/errors.go b/iwf/errors.go
index e201ae8..489e8c7 100644
--- a/iwf/errors.go
+++ b/iwf/errors.go
@@ -171,7 +171,7 @@ func IsWorkflowNotExistsError(err error) bool {
 
 func IsRPCError(err error) bool {
 	apiError, ok := err.(*ApiError)
-	if !ok || apiError.Response == nil {
+	if !ok {
 		return false
 	}
 	return apiError.StatusCode == 420