Fix gRPC ObjC void function definition
diff --git a/src/objective-c/RxLibrary/GRXImmediateWriter.h b/src/objective-c/RxLibrary/GRXImmediateWriter.h
index bdcf5d5..f88e46b 100644
--- a/src/objective-c/RxLibrary/GRXImmediateWriter.h
+++ b/src/objective-c/RxLibrary/GRXImmediateWriter.h
@@ -46,7 +46,7 @@
* Returns a writer that pushes to its writeable the successive values returned by the passed
* block. When the block first returns nil, it is released.
*/
-+ (GRXWriter *)writerWithValueSupplier:(id (^)())block;
++ (GRXWriter *)writerWithValueSupplier:(id (^)(void))block;
/**
* Returns a writer that iterates over the values of the passed container and pushes them to