diff -ur mdk3-v5-orig/mdk3.c mdk3-v5/mdk3.c
--- mdk3-v5-orig/mdk3.c	2008-11-10 21:45:02.000000000 +0100
+++ mdk3-v5/mdk3.c	2009-01-10 01:47:03.546875000 +0100
@@ -1092,10 +1092,13 @@
 {
   struct clistwidsclient *first = c;
 
-  do {
-    if (!(memcmp(c->mac, mac, mac_len))) return c;
-    c = c->next;
-  } while (c != first);
+  if (c != NULL)
+  {
+    do {
+      if (c->mac != NULL && !(memcmp(c->mac, mac, mac_len))) return c;
+      c = c->next;
+    } while (c != NULL && c != first);
+  }
 
   return NULL;
 }