diff --git a/dix/ffs.c b/dix/ffs.c
index e25d715dd..80fbcc8a9 100644
--- a/dix/ffs.c
+++ b/dix/ffs.c
@@ -30,6 +30,8 @@ int
 ffs(int i)
 {
     int j;
+    if (i == 0)
+        return 0;
     for (j = 1; (i & 1) == 0; j++)
 	i >>= 1;
     return j;
diff --git a/include/dix.h b/include/dix.h
index f346b43a2..772d65e7c 100644
--- a/include/dix.h
+++ b/include/dix.h
@@ -815,4 +815,7 @@ typedef struct {
 extern int xstrcasecmp(char *s1, char *s2);
 #endif
 
+/* ffs.c */
+extern int ffs(int i);
+
 #endif /* DIX_H */