diff --git a/gfx/layers/composite/LayerManagerComposite.cpp b/gfx/layers/composite/LayerManagerComposite.cpp
index 0e8744a2491..cc5dabe4994 100644
--- a/gfx/layers/composite/LayerManagerComposite.cpp
+++ b/gfx/layers/composite/LayerManagerComposite.cpp
@@ -417,7 +417,7 @@ LayerManagerComposite::RenderDebugOverlay(const Rect& aBounds)
     // Draw a translation delay warning overlay
     int width;
     int border;
-    if ((now - mWarnTime).ToMilliseconds() < kVisualWarningDuration) {
+    if (!mWarnTime.IsNull() && (now - mWarnTime).ToMilliseconds() < kVisualWarningDuration) {
       EffectChain effects;
 
       // Black blorder