diff --git a/src/command_func.h b/src/command_func.h
--- a/src/command_func.h
+++ b/src/command_func.h
@@ -77,6 +77,7 @@ void NetworkSend_Command(TileIndex tile,
  */
 extern const char *_cmd_text;
 extern Money _additional_cash_required;
+extern StringID _error_message;
 
 /**
  * Checks if a integer value belongs to a command.