diff --git a/ssl/pqueue/pqueue.c b/ssl/pqueue/pqueue.c
index ecaa139..14bd9b6 100644
--- a/ssl/pqueue/pqueue.c
+++ b/ssl/pqueue/pqueue.c
@@ -56,6 +56,7 @@
 
 #include <openssl/pqueue.h>
 
+#include <assert.h>
 #include <string.h>
 
 #include <openssl/mem.h>
@@ -104,6 +105,8 @@
     return;
   }
 
+  /* The queue must be empty. */
+  assert(pq->items == NULL);
   OPENSSL_free(pq);
 }