Example: user=> (if-let [foo (+ 1 2)] foo + 1 foo) java.lang.IllegalArgumentException: if-let requires a vector for its binding (NO_SOURCE_FILE:2)

The error reports that if-let requires a vector for its binding, but the problem here has nothing to do with the binding being a vector. A paren was forgotten before the '+', so the number of arguments is incorrect for the if form.