If your issue revolves around the use of a particular package; please please please, say what package you are using.
multinum appears both in mgcv package, as well as nnet; perhaps others; I assume that you are using nnet.
Your problem boils down to overzealous use of $ syntax within multinom formula; formulas are great because they can assume the context; the context of the data = you gave. At best in R functions that take formulas the repetition of some_data_source$x etc. serves no advantage, but as in your case, it can often literally cause the functions to bug out.