diff --git a/packages/grpc-native-core/index.d.ts b/packages/grpc-native-core/index.d.ts
index 5a48accb7..fe623b8bb 100644
--- a/packages/grpc-native-core/index.d.ts
+++ b/packages/grpc-native-core/index.d.ts
@@ -312,7 +312,7 @@ declare module "grpc" {
   /**
    * An object that completely defines a service.
    */
-  export type ServiceDefinition<ImplementationType> = {
+  export type ServiceDefinition<ImplementationType = UntypedServiceImplementation> = {
     readonly [I in keyof ImplementationType]: MethodDefinition<any, any>;
   }