diff --git a/test/unit/lwip_unittests.c b/test/unit/lwip_unittests.c
index 3cb4a885..5b8d523f 100644
--- a/test/unit/lwip_unittests.c
+++ b/test/unit/lwip_unittests.c
@@ -50,7 +50,8 @@ void lwip_check_ensure_no_alloc(unsigned int skip)
   unsigned int mask;
 
   if (!(skip & SKIP_HEAP)) {
-    fail_unless(lwip_stats.mem.used == 0);
+    fail_unless(lwip_stats.mem.used == 0,
+      "mem heap still has %d bytes allocated", lwip_stats.mem.used);
   }
   for (i = 0, mask = 1; i < MEMP_MAX; i++, mask <<= 1) {
     if (!(skip & mask)) {