--- a/layout/base/nsLayoutUtils.cpp
+++ b/layout/base/nsLayoutUtils.cpp
@@ -1908,18 +1908,21 @@ nsLayoutUtils::DoCompareTreePosition(nsI
 
   nsPresContext* presContext = aFrame1->PresContext();
   if (presContext != aFrame2->PresContext()) {
     NS_ERROR("no common ancestor at all, different documents");
     return 0;
   }
 
   AutoTArray<nsIFrame*,20> frame1Ancestors;
-  if (aCommonAncestor &&
-      !FillAncestors(aFrame1, aCommonAncestor, &frame1Ancestors)) {
+  // Note that the order of the condition is important. We need to fill the
+  // ancestors even if aCommonAncestor is null, otherwise the code below makes
+  // no sense.
+  if (!FillAncestors(aFrame1, aCommonAncestor, &frame1Ancestors) &&
+      aCommonAncestor) {
     // We reached the root of the frame tree ... if aCommonAncestor was set,
     // it is wrong
     return DoCompareTreePosition(aFrame1, aFrame2,
                                  aIf1Ancestor, aIf2Ancestor, nullptr);
   }
 
   int32_t last1 = int32_t(frame1Ancestors.Length()) - 1;
   int32_t last2 = int32_t(aFrame2Ancestors.Length()) - 1;