--- /dev/null
+diff --git a/hw/xfree86/modes/xf86Crtc.h b/hw/xfree86/modes/xf86Crtc.h
+index 802303f..6e80210 100644
+--- a/hw/xfree86/modes/xf86Crtc.h
++++ b/hw/xfree86/modes/xf86Crtc.h
+@@ -730,6 +730,8 @@ static _X_INLINE xf86OutputPtr
+ xf86CompatOutput(ScrnInfoPtr pScrn)
+ {
+ xf86CrtcConfigPtr config = XF86_CRTC_CONFIG_PTR(pScrn);
++ if (config->compat_output < 0)
++ return NULL;
+
+ return config->output[config->compat_output];
+ }