When you call a Keyword with the wrong number of arguments, the error message does not report how many arguments were passed:
compare to calling an IFn, which does show the number of arguments passed:
The latter error message is more clear and makes it easier to debug.
The attached patch re-uses the ArityException class used elsewhere to generate error messages in the latter form when calling a keyword with the wrong number of arguments.
Prescreened by: Alex Miller