diff --git a/lib/libmthread/Makefile b/lib/libmthread/Makefile
index 5e8c3b7d8..a35cc5e64 100644
--- a/lib/libmthread/Makefile
+++ b/lib/libmthread/Makefile
@@ -1,6 +1,6 @@
 # Makefile for libmthread
 
-CPPFLAGS+=-O -D_MINIX -D_POSIX_SOURCE -Wall
+CPPFLAGS+=-O -D_MINIX -D_POSIX_SOURCE -Wall -Werror
 
 LIB=	mthread
 
diff --git a/lib/libmthread/misc.c b/lib/libmthread/misc.c
index 81301f31b..57e94bb39 100644
--- a/lib/libmthread/misc.c
+++ b/lib/libmthread/misc.c
@@ -21,11 +21,15 @@ PUBLIC void mthread_debug_f(const char *file, int line, const char *msg)
 PUBLIC void mthread_panic_f(const char *file, int line, const char *msg)
 {
   /* Print panic message to stdout and exit */
+  volatile int *sf;
+
+  sf = NULL;
+
   printf("mthread panic (%s:%d): ", file, line);
-  printf(msg);
+  printf("%s", msg);
   printf("\n");
   fflush(stdout);	/* Force debug print to screen */
-  *((int *)0) = 1;	/* Cause segfault to generate trace */
+  *((int *) sf ) = 1;	/* Cause segfault to generate trace */
   exit(1);
 }
 
diff --git a/lib/libmthread/mutex.c b/lib/libmthread/mutex.c
index 5aacbcf03..0515556b9 100644
--- a/lib/libmthread/mutex.c
+++ b/lib/libmthread/mutex.c
@@ -190,6 +190,8 @@ mthread_mutex_t *mutex;	/* Mutex that is to be locked */
   m = (struct __mthread_mutex *) *mutex;
   if (!mthread_mutex_valid(&m))
   	return(EINVAL);
+  else if (m->mm_owner == current_thread)
+	return(EDEADLK);
   else if (m->mm_owner == NO_THREAD) {
 	m->mm_owner = current_thread;
 	return(0);