This self-contained .drl runs fine with 5.3.0, crashes (with two different exceptions) in 5.[45].0, and is back to OK with 6.0.0. package decl declare Cont answer : String end function boolean isValid( String s ){ try { Double.valueOf( s ); return true; } catch( NumberFormatException nfe ){ return false; } } rule isString when $s: Cont( $a: answer, isValid( answer ) ) then System.out.println( "isString: " + $a ); end rule isX when then insert( new Cont( "123.456" ) ); end