diff --git a/src/objective-c/RxLibrary/GRXForwardingWriter.m b/src/objective-c/RxLibrary/GRXForwardingWriter.m
index cf8be8c557e..a72be9ace2f 100644
--- a/src/objective-c/RxLibrary/GRXForwardingWriter.m
+++ b/src/objective-c/RxLibrary/GRXForwardingWriter.m
@@ -51,7 +51,8 @@
     return nil;
   }
   if (writer.state != GRXWriterStateNotStarted) {
-    [NSException raise:NSInvalidArgumentException format:@"writer can't be started."];
+    [NSException raise:NSInvalidArgumentException
+                format:@"The writer argument must not have already started."];
   }
   if ((self = [super init])) {
     _writer = writer;