diff --git a/src/objective-c/ProtoRPC/ProtoRPC.h b/src/objective-c/ProtoRPC/ProtoRPC.h
index 635ba0c90ee..b0f4ced99e8 100644
--- a/src/objective-c/ProtoRPC/ProtoRPC.h
+++ b/src/objective-c/ProtoRPC/ProtoRPC.h
@@ -134,10 +134,11 @@ __attribute__((deprecated("Please use GRPCProtoCall."))) @interface ProtoRPC
        * addr and the port number, for example @"localhost:5050".
        */
       -
-      (instancetype)initWithHost : (NSString *)host method
-    : (GRPCProtoMethod *)method requestsWriter : (GRXWriter *)requestsWriter responseClass
-    : (Class)responseClass responsesWriteable
-    : (id<GRXWriteable>)responsesWriteable NS_DESIGNATED_INITIALIZER;
+      (instancetype _Null_unspecified)initWithHost : (NSString *_Null_unspecified)host method
+    : (GRPCProtoMethod *_Null_unspecified)method requestsWriter
+    : (GRXWriter *_Null_unspecified)requestsWriter responseClass
+    : (Class _Null_unspecified)responseClass responsesWriteable
+    : (id<GRXWriteable> _Null_unspecified)responsesWriteable NS_DESIGNATED_INITIALIZER;
 
 - (void)start;
 @end