I can concede that it may depend on the precise formal system you are using, but I think you can easily imagine a logic where a predicate ∈ takes two variables (x, y) of any kind and returns "true" iff y is a set and x is in y. In a well-founded system where numbers are ur-elements ∈(5, 5) would be well-formed and equal to "false" as every other expression of the form ∈(x, x).
In this vein I would consider "5 smells insanity" nonsencial in the colloquial sense and at the same time false.
