Hi, I can help with this one. The problem is in the top row directly under "Stack trace:" line. Problem was encountered in REPLACE instruction. (probably Source argument was invalid. You can try to write to console content of Source argument before you call REPLACE. The problematic REPLACE was in VS defined in "messages and text" Game logic (with ID=225) Steps should be: Open scripts for "messages and text" Game logic (with ID=225) Write "#237" to search and press enter. It should focus specific REPLACE instruction (needed in complicated pieces of code) Try to find out if any of REPLACE arguments can be invalid. Regarding other lines in stack trace: they should help you to understand how the issue happened. In this case you should go from the bottom to the top line. On Trigger enter was triggered in forest hidden object trigger zone (ID:523) There was call of your custom instruction "base skill check" from the same trigger zone From there it went through your "return attribute value" instruction defined in skillCheck game logic object and you probably get the idea at the moment. Line+Column info can help advance users who check text script which can be found next to output_log_clean.log (userscript.js) when you playtest your game. If you do not feel to look into text scripts then you can ignore this part.