diff --git a/os/macosx/macos.m b/os/macosx/macos.m
--- a/os/macosx/macos.m
+++ b/os/macosx/macos.m
@@ -13,3 +13,14 @@ void ShowMacDialog ( const char *title, 
 	NSRunAlertPanel([NSString stringWithCString: title], [NSString stringWithCString: message], [NSString stringWithCString: buttonLabel], nil, nil);
 }
 
+void ShowMacAssertDialog ( const char *function, const char *file, const int line, const char *expression )
+{
+	const char *buffer = 
+			[[NSString stringWithFormat:@"An assertion has failed and OpenTTD must quit.\n%s in %s (line %d)\n\"%s\"\n\nYou should report this error the OpenTTD developers if you think you found a bug.",
+			function, file, line, expression] cStringUsingEncoding:NSASCIIStringEncoding];
+	NSLog(@"%s", buffer);
+	ShowMacDialog( "Assertion Failed", buffer, "Quit" );
+	
+	// abort so that a debugger has a chance to notice
+	abort();
+}