--- a/layout/base/nsCSSFrameConstructor.cpp
+++ b/layout/base/nsCSSFrameConstructor.cpp
@@ -6508,20 +6508,17 @@ nsCSSFrameConstructor::AppendFramesToPar
 // inside fieldsets, (5) popups and other kids of the menu are siblings from a
 // content perspective, they are not considered siblings in the frame tree.
 bool
 nsCSSFrameConstructor::IsValidSibling(nsIFrame*              aSibling,
                                       nsIContent*            aContent,
                                       StyleDisplay&          aDisplay)
 {
   nsIFrame* parentFrame = aSibling->GetParent();
-  nsIAtom* parentType = nullptr;
-  if (parentFrame) {
-    parentType = parentFrame->GetType();
-  }
+  nsIAtom* parentType = parentFrame->GetType();
 
   StyleDisplay siblingDisplay = aSibling->GetDisplay();
   if (StyleDisplay::TableColumnGroup == siblingDisplay ||
       StyleDisplay::TableColumn      == siblingDisplay ||
       StyleDisplay::TableCaption     == siblingDisplay ||
       StyleDisplay::TableHeaderGroup == siblingDisplay ||
       StyleDisplay::TableRowGroup    == siblingDisplay ||
       StyleDisplay::TableFooterGroup == siblingDisplay ||