Maybe there are too many parameters in the send call when you're adding items? Send/self/super calls put a sequence of values on the stack, of the form:
selectorNumber numParams [param0-paramN] selectorNumber numParams [param0-paramN] ... totalNumberOfBytes
I'm guessing something is wrong somewhere in there and one of the parameters is being treated as a selector number (method call).
What does your templateInventory init method look like? More specifically, the self call when you add the items? What does the disassembly look like? (Script -> Disassemble).
Also, what's the size the size of the heap and script resources for this script? (script 15, I presume). Can you attach the script and heap resources to a post?