diff --git a/cmake/lws_config.h.in b/cmake/lws_config.h.in
index 394651acb..69d187758 100644
--- a/cmake/lws_config.h.in
+++ b/cmake/lws_config.h.in
@@ -179,6 +179,7 @@
 #cmakedefine LWS_WITH_STATS
 #cmakedefine LWS_WITH_STRUCT_SQLITE3
 #cmakedefine LWS_WITH_STRUCT_JSON
+#cmakedefine LWS_WITH_SUL_DEBUGGING
 #cmakedefine LWS_WITH_SQLITE3
 #cmakedefine LWS_WITH_SYS_NTPCLIENT
 #cmakedefine LWS_WITH_SYS_DHCP_CLIENT
diff --git a/lib/core-net/sorted-usec-list.c b/lib/core-net/sorted-usec-list.c
index 080bd3518..4b3122122 100644
--- a/lib/core-net/sorted-usec-list.c
+++ b/lib/core-net/sorted-usec-list.c
@@ -337,10 +337,12 @@ lws_sul_debug_zombies(struct lws_context *ctx, void *po, size_t len,
 				 * indicated as being deleted?
 				 */
 
-				if (sul >= po && lws_ptr_diff(sul, po) < len) {
+				if ((void *)sul >= po &&
+				    (size_t)lws_ptr_diff(sul, po) < len) {
 					lwsl_err("%s: ERROR: Zombie Sul "
-						 "(on list %d) %s\n", __func__,
-						 m, destroy_description);
+						 "(on list %d) %s, cb %p\n",
+						 __func__, m,
+						 destroy_description, sul->cb);
 					/*
 					 * This assert fires if you have left
 					 * a sul scheduled to fire later, but
@@ -350,6 +352,12 @@ lws_sul_debug_zombies(struct lws_context *ctx, void *po, size_t len,
 					 * that may be scheduled before
 					 * destroying the object the sul lives
 					 * inside.
+					 *
+					 * You can look up the cb pointer in
+					 * your mapfile to find out which
+					 * callback function the sul was using
+					 * which usually tells you which sul
+					 * it is.
 					 */
 					assert(0);
 				}