diff --git a/glx/glxcmds.c b/glx/glxcmds.c
index 009fd9be0..ea42e2a01 100644
--- a/glx/glxcmds.c
+++ b/glx/glxcmds.c
@@ -2062,7 +2062,7 @@ __glXDisp_Render(__GLXclientState * cl, GLbyte * pc)
             extra = (*entry.varsize) (pc + __GLX_RENDER_HDR_SIZE,
                                       client->swapped);
             if (extra < 0) {
-                extra = 0;
+                return BadLength;
             }
             if (cmdlen != __GLX_PAD(entry.bytes + extra)) {
                 return BadLength;
@@ -2179,7 +2179,7 @@ __glXDisp_RenderLarge(__GLXclientState * cl, GLbyte * pc)
             extra = (*entry.varsize) (pc + __GLX_RENDER_LARGE_HDR_SIZE,
                                       client->swapped);
             if (extra < 0) {
-                extra = 0;
+                return BadLength;
             }
             /* large command's header is 4 bytes longer, so add 4 */
             if (cmdlen != __GLX_PAD(entry.bytes + 4 + extra)) {