IE does support the toString method. However, your load rule may fail if it attempts to do a toString on an undefined variable e.g. if your load rule says "page_name contains ignore case 'test'", then if page_name is not defined, you'll get an uncaught error:
"Cannot call method toString of undefined" or something similar.
To avoid this, it's best to add a check to the load rule that the variable is defined before trying to perform a comparison on it:
"page_name IS DEFINED
AND page_name contains_ignore_case 'test'".
I hope this helps to explain what's going on.
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