javakey does not report the exception that causes an error when it generates an null signature while trying to sign a jar file. The error message reads:

error generating signature: null[null]

it should read:

error generating signature: name.of.exception[null]

the reason for this is an incorrect call to error() in sun.security.provider.Main.

This bug also exists in e_p_java and has been filed as 4140855, it will be fixed in version pjava1.1v.