Comment #1 on issue 618 by pekka.klarck: Java long and Long parameters are passed as BigInteger
http://code.google.com/p/robotframework/issues/detail?id=618
What kind of signature your Java keyword has? Jython coerces arguments when Java method is called, so it wouldn't be a surprise if a long was converted to a BigInteger if that's what your Java method has in it's signature.
