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understanding I() in lmer formula

The expression `~z.n.males` is a unary formula, i.e. a language object, so it's not a data object much less a numeric data object. The `*` operator does not have a method for such an object as the second argument.

It appears that trying to use I() on an expression with a multiplication operator was not something that was anticipated as having a sensible meaning.

I'm not sure what this was supposed to be doing but perhaps you wanted the interaction()-function rather than the as.is function?