diff --git a/lib/linklist.c b/lib/linklist.c
index 5a2b696..3cb10ca 100644
--- a/lib/linklist.c
+++ b/lib/linklist.c
@@ -162,6 +162,7 @@
 {
   struct listnode *node;
 
+  assert(list);
   for (node = list->head; node; node = node->next)
     {
       if (node->data == val)
@@ -189,6 +190,7 @@
 {
   struct listnode *node;
 
+  assert(list);
   node = list->head;
 
   if (node)
@@ -203,6 +205,7 @@
   struct listnode *node;
   struct listnode *next;
 
+  assert(list);
   for (node = list->head; node; node = next)
     {
       next = node->next;
@@ -221,6 +224,7 @@
   struct listnode *node;
   struct listnode *next;
 
+  assert(list);
   for (node = list->head; node; node = next)
     {
       next = node->next;
@@ -237,6 +241,7 @@
 {
   listnode node;
 
+  assert(list);
   for (node = list->head; node; nextnode (node))
     if (data == getdata (node))
       return node;