using this script: def foo(Number s){} def foo(int i ){} foo() an exception is thrown whcih looks like: Caught: groovy.lang.GroovyRuntimeException: Could not find which method to invoke from this list: org.codehaus.groovy.reflection.FastArray@9df354 for arguments: {} groovy.lang.GroovyRuntimeException: Could not find which method to invoke from this list: org.codehaus.groovy.reflection.FastArray@9df354 for arguments: {} the org.codehaus.groovy.reflection.FastArray@9df354 is wrong, at his place there should be a list of the available methods of that name that is foo(Number) and foo(int), but no toString default implementation. This text is important for the user to find an error.