diff --git a/src/objective-c/ProtoRPC/ProtoRPC.h b/src/objective-c/ProtoRPC/ProtoRPC.h
index bd926b732873efb887aced402518612f6ead9db5..642b8890ffbd4221ae529d30e3e52d58c7a858bd 100644
--- a/src/objective-c/ProtoRPC/ProtoRPC.h
+++ b/src/objective-c/ProtoRPC/ProtoRPC.h
@@ -38,6 +38,10 @@
 
 @interface ProtoRPC : GRPCCall
 
+/*
+ * host parameter should not contain the scheme (http:// or https://), only the name or IP addr
+ * and the port number, for example @"localhost:5050".
+ */
 - (instancetype)initWithHost:(NSString *)host
                       method:(ProtoMethod *)method
               requestsWriter:(GRXWriter *)requestsWriter