Rule Engine: Comment throws error


I thought this has been fixed but it seems a comment causes:

failed to apply settings: Ambiguous return type! Use 'string()' or another function from the "Convert Type" category to specify the return type.

// Filter identical URLs whos lastmod date is older or equal
join("$URL$ = \"", $URL$, "\" AND $lastmod$ <= \"", $lastmod$, "\" => TRUE")

Removing the comment make the node executable: