The keyword function should throw an IllegalArgumentException on wrong argument type rather than return nil. The two-argument case already throws an exception.
The find-keyword function should behave similarly to maintain the same signature.
user=> (keyword 5)
user=> (keyword )
user=> (keyword 1 1)
java.lang.ClassCastException: java.lang.Long cannot be cast to java.lang.String
Cause: The keyword function is defined as a cond with cases for keywords, symbols, and strings. There is no :else so all other cases return nil. The same goes for find-keyword, which should behave in the same way.
Proposal: I have added an :else branch to the cond that throws an IllegalArgumentException with a message that indicates the acceptable types and prints the actual argument. I made the same change to find-keyword. There are also simple tests.
Alternatives: Adding checks for the two-argument case was considered but it was feared that adding the extra overhead was not worth it since it already threw an exception. No significant overhead is added in the single-argument case since it will only affect erroneous input.
Note: This change does not check for all bad input, just the type. For instance, it is still possible to pass in a string with "illegal" keyword characters.