diff --git a/src/threads.c b/src/threads.c
index 76e0535596d57eb7b5b89817de5daa199a02b039..d1863486f487bf8926937a007899c61ff4d553f3 100644
--- a/src/threads.c
+++ b/src/threads.c
@@ -1,5 +1,5 @@
 #include "global.h"
-RCSID("$Id: threads.c,v 1.100 1999/06/19 20:18:43 hubbe Exp $");
+RCSID("$Id: threads.c,v 1.101 1999/07/01 01:39:13 hubbe Exp $");
 
 int num_threads = 1;
 int threads_disabled = 0;
@@ -746,6 +746,11 @@ void f_mutex_lock(INT32 args)
 
   if(m->key)
   {
+    if(threads_disabled)
+    {
+      free_object(o);
+      error("Cannot wait for mutexes when threads are disabled!\n");
+    }
     SWAP_OUT_CURRENT_THREAD();
     do
     {
@@ -900,6 +905,9 @@ void f_cond_wait(INT32 args)
     mut=OB2KEY(key)->mut;
     if(!mut) error("Bad argument 1 to condition->wait()\n");
 
+    if(threads_disabled)
+      error("Cannot wait for conditions when threads are disabled!\n");
+
     /* Unlock mutex */
     mut->key=0;
     OB2KEY(key)->mut=0;