Please email me a sample project that causes this. From the code this looks impossible - that variable is assigned 60 lines up in the same function and the dictionary does unconditionally get a "set" function.
Turns out that this is a GM bug! If you have
self.a_resource_name = value; var v = self.a_resource_name;
then you get a resource ID back instead of the variable value. If you find-replace "macros" in the GMLive script into something else (or rename the "macros" script), it'll work fine. I'll rename this for the next version since name "macros" is common enough.