I made info on my receivers to closely resemble ERM Help:

Quote:If a command parameter is shown as $ it means that its value can be stored in a variable. If it's shown as # it means that it can only be set, but not read. And if it's shown as ?$ it means it can be read but not set.

So the first command alone should cause error and produce a message with a text of the faulty command, like your ordinary ERM error would. I am yet to implement error messages, Era's ExecErmCmd proved surprisingly incapable of handling IF:M during my earlier tests.
Now the reason why I implemented getting creatures/artifacts this way is that you can set unlimited number of them to be delivered and I consider LE:A-like implementation to be lame. So you must cycle through all artifacts, get quantity of each and compose your message.
Why it sometimes works for you I don't know. Probably ERM recovers from error (since I provide valid recovery info) and continues to run with faulty command unexecuted and values unchanged.