--- a/layout/generic/nsFlexContainerFrame.cpp
+++ b/layout/generic/nsFlexContainerFrame.cpp
@@ -4515,20 +4515,20 @@ nsFlexContainerFrame::DoFlexLayout(nsPre
 
         WritingMode wm = item->Frame()->GetWritingMode();
         LogicalSize availSize = aReflowInput.ComputedSize(wm);
         availSize.BSize(wm) = NS_UNCONSTRAINEDSIZE;
         ReflowInput childReflowInput(aPresContext, aReflowInput,
                                      item->Frame(), availSize);
         if (!sizeOverride) {
           // Directly override the computed main-size, by tweaking reflow state:
-          if (aAxisTracker.IsMainAxisHorizontal()) {
-            childReflowInput.SetComputedWidth(item->GetMainSize());
+          if (item->IsInlineAxisMainAxis()) {
+            childReflowInput.SetComputedISize(item->GetMainSize());
           } else {
-            childReflowInput.SetComputedHeight(item->GetMainSize());
+            childReflowInput.SetComputedBSize(item->GetMainSize());
           }
         }
 
         SizeItemInCrossAxis(aPresContext, aAxisTracker,
                             childReflowInput, *item);
       }
     }
     // Now that we've finished with this line's items, size the line itself: