diff --git a/lib/alloca.c b/lib/alloca.c
index 55a80677..836c5031 100644
--- a/lib/alloca.c
+++ b/lib/alloca.c
@@ -88,6 +88,7 @@ typedef char *pointer;
    Callers below should use malloc.  */
 
 #  ifndef emacs
+#   undef malloc
 #   define malloc xmalloc
 #  endif
 extern pointer malloc ();