diff --git a/src/xcb_out.c b/src/xcb_out.c
index 054a2ae..78ec8ca 100644
--- a/src/xcb_out.c
+++ b/src/xcb_out.c
@@ -87,6 +87,7 @@ int XCBSendRequest(XCBConnection *c, unsigned int *request, int flags, struct io
         int i;
         CARD16 shortlen = 0;
         size_t longlen = 0;
+        assert(vector[0].iov_len >= 4);
         /* set the major opcode, and the minor opcode for extensions */
         if(req->ext)
         {