diff --git a/module/mes/scm.mes b/module/mes/scm.mes
index 3aaac56d..9beac54b 100644
--- a/module/mes/scm.mes
+++ b/module/mes/scm.mes
@@ -109,7 +109,7 @@
 (define assv-ref assq-ref)
 
 (define (assoc key alist)
-  (if (null? alist) #f
+  (if (not (pair? alist)) #f
       (if (equal? key (caar alist)) (car alist)
           (assoc key (cdr alist)))))
 
diff --git a/src/mes.c b/src/mes.c
index 46544f70..c0aae433 100644
--- a/src/mes.c
+++ b/src/mes.c
@@ -713,6 +713,8 @@ call (SCM fn, SCM x)
 SCM
 assq (SCM x, SCM a)
 {
+  if (TYPE (a) != TPAIR)
+    return cell_f;
   switch (TYPE (x))
     {
     case TCHAR: