--- a/layout/svg/nsSVGIntegrationUtils.cpp
+++ b/layout/svg/nsSVGIntegrationUtils.cpp
@@ -137,16 +137,18 @@ GetPreEffectsVisualOverflowUnion(nsIFram
   return collector.GetResult() + aFirstContinuationToUserSpace;
 }
 
 static nsRect
 GetPreEffectsVisualOverflow(nsIFrame* aFirstContinuation,
                             nsIFrame* aCurrentFrame,
                             const nsPoint& aFirstContinuationToUserSpace)
 {
+  MOZ_ASSERT(aFirstContinuation ==
+             nsLayoutUtils::FirstContinuationOrIBSplitSibling(aCurrentFrame));
   PreEffectsVisualOverflowCollector collector(aFirstContinuation,
                                               nullptr,
                                               nsRect(),
                                               false);
   // Compute overflow areas of current frame relative to aFirstContinuation:
   nsLayoutUtils::AddBoxesForFrame(aCurrentFrame, &collector);
   // Return the result in user space:
   return collector.GetResult() + aFirstContinuationToUserSpace;