--- a/layout/generic/nsFlexContainerFrame.cpp
+++ b/layout/generic/nsFlexContainerFrame.cpp
@@ -1498,26 +1498,27 @@ MainAxisPositionTracker::
 {
   MOZ_ASSERT(aReflowState.frame == aFlexContainerFrame,
              "Expecting the reflow state for the flex container frame");
 
   // mPackingSpaceRemaining is initialized to the container's main size.  Now
   // we'll subtract out the main sizes of our flex items, so that it ends up
   // with the *actual* amount of packing space.
   for (uint32_t i = 0; i < aItems.Length(); i++) {
+    const FlexItem& curItem = aItems[i];
     nscoord itemMarginBoxMainSize =
-      aItems[i].GetMainSize() +
-      aItems[i].GetMarginBorderPaddingSizeInAxis(aAxisTracker.GetMainAxis());
+      curItem.GetMainSize() +
+      curItem.GetMarginBorderPaddingSizeInAxis(aAxisTracker.GetMainAxis());
     mPackingSpaceRemaining -= itemMarginBoxMainSize;
+    mNumAutoMarginsInMainAxis += curItem.GetNumAutoMarginsInAxis(mAxis);
   }
 
-  if (mPackingSpaceRemaining > 0) {
-    for (uint32_t i = 0; i < aItems.Length(); i++) {
-      mNumAutoMarginsInMainAxis += aItems[i].GetNumAutoMarginsInAxis(mAxis);
-    }
+  if (mPackingSpaceRemaining <= 0) {
+    // No available packing space to use for resolving auto margins.
+    mNumAutoMarginsInMainAxis = 0;
   }
 
   mJustifyContent = aFlexContainerFrame->StylePosition()->mJustifyContent;
   // If packing space is negative, 'justify' behaves like 'start', and
   // 'distribute' behaves like 'center'.  In those cases, it's simplest to
   // just pretend we have a different 'justify-content' value and share code.
   if (mPackingSpaceRemaining < 0) {
     if (mJustifyContent == NS_STYLE_JUSTIFY_CONTENT_SPACE_BETWEEN) {