--- a/layout/base/nsLayoutUtils.cpp
+++ b/layout/base/nsLayoutUtils.cpp
@@ -1471,17 +1471,17 @@ nsLayoutUtils::GetClosestFrameOfType(nsI
 }
 
 // static
 nsIFrame*
 nsLayoutUtils::GetStyleFrame(nsIFrame* aFrame)
 {
   if (aFrame->GetType() == nsGkAtoms::tableOuterFrame) {
     nsIFrame* inner = aFrame->PrincipalChildList().FirstChild();
-    NS_ASSERTION(inner, "Outer table must have an inner");
+    // inner may be null, if aFrame is mid-destruction
     return inner;
   }
 
   return aFrame;
 }
 
 nsIFrame*
 nsLayoutUtils::GetStyleFrame(const nsIContent* aContent)

--- a/layout/base/nsLayoutUtils.h
+++ b/layout/base/nsLayoutUtils.h
@@ -326,16 +326,20 @@ public:
     return GetClosestFrameOfType(aFrame, nsGkAtoms::pageFrame);
   }
 
   /**
    * Given a frame which is the primary frame for an element,
    * return the frame that has the non-psuedoelement style context for
    * the content.
    * This is aPrimaryFrame itself except for tableOuter frames.
+   *
+   * Given a non-null input, this will return null if and only if its
+   * argument is a table outer frame that is mid-destruction (and its
+   * table frame has been destroyed).
    */
   static nsIFrame* GetStyleFrame(nsIFrame* aPrimaryFrame);
 
   /**
    * Given a content node,
    * return the frame that has the non-psuedoelement style context for
    * the content.  May return null.
    * This is aContent->GetPrimaryFrame() except for tableOuter frames.