diff --git a/util/ack/ack.h b/util/ack/ack.h
index 0153e9947..4a2c98f41 100644
--- a/util/ack/ack.h
+++ b/util/ack/ack.h
@@ -7,7 +7,7 @@
 /****************************************************************************/
 
 #define FRONTENDS       "fe"    /* The front-end definitions */
-#define TMPNAME         "Ack%04x"     /* Naming of temp. files */
+#define TMPNAME         "Ack_%x"     /* Naming of temp. files */
 
 /****************************************************************************/
 /*         Internal mnemonics, should not be tinkered with                  */