--- a/layout/generic/nsFlexContainerFrame.cpp
+++ b/layout/generic/nsFlexContainerFrame.cpp
@@ -1044,17 +1044,19 @@ FlexItem::FlexItem(nsIFrame* aChildFrame
     mIsFrozen(false),
     mHadMinViolation(false),
     mHadMaxViolation(false),
     mHadMeasuringReflow(false),
     mIsStretched(false),
     mIsStrut(false),
     mAlignSelf(aChildFrame->StylePosition()->mAlignSelf)
 {
-  MOZ_ASSERT(aChildFrame, "expecting a non-null child frame");
+  MOZ_ASSERT(mFrame, "expecting a non-null child frame");
+  MOZ_ASSERT(!mFrame->IsAbsolutelyPositioned(),
+             "abspos child frames should not be treated as flex items");
 
   SetFlexBaseSizeAndMainSize(aFlexBaseSize);
 
   // Assert that any "auto" margin components are set to 0.
   // (We'll resolve them later; until then, we want to treat them as 0-sized.)
 #ifdef DEBUG
   {
     const nsStyleSides& styleMargin = mFrame->StyleMargin()->mMargin;
@@ -1112,19 +1114,22 @@ FlexItem::FlexItem(nsIFrame* aChildFrame
     mIsFrozen(true),
     mHadMinViolation(false),
     mHadMaxViolation(false),
     mHadMeasuringReflow(false),
     mIsStretched(false),
     mIsStrut(true), // (this is the constructor for making struts, after all)
     mAlignSelf(NS_STYLE_ALIGN_ITEMS_FLEX_START)
 {
+  MOZ_ASSERT(mFrame, "expecting a non-null child frame");
   MOZ_ASSERT(NS_STYLE_VISIBILITY_COLLAPSE ==
              mFrame->StyleVisibility()->mVisible,
              "Should only make struts for children with 'visibility:collapse'");
+  MOZ_ASSERT(!mFrame->IsAbsolutelyPositioned(),
+             "abspos child frames should not be treated as flex items");
 }
 
 nscoord
 FlexItem::GetBaselineOffsetFromOuterCrossStart(
   AxisOrientationType aCrossAxis) const
 {
   // NOTE: Currently, 'mAscent' (taken from reflow) is an inherently vertical
   // measurement -- it's the distance from the border-top edge of this FlexItem