new file mode 100644
--- /dev/null
+++ b/layout/forms/crashtests/1600367.html
@@ -0,0 +1,8 @@
+<style>
+FIELDSET {
+  border: 35482ex groove ! important;
+  all: unset;
+}
+</style>
+<i style="font-size-adjust: 113;" readonly>
+<fieldset>

--- a/layout/forms/crashtests/crashtests.list
+++ b/layout/forms/crashtests/crashtests.list
@@ -68,8 +68,9 @@ load 1388230-1.html
 load 1388230-2.html
 load 1405830.html
 load 1418477.html
 load 1432853.html
 asserts(1-4) load 1460787-1.html
 load 1464165-1.html
 load 1471157.html
 load 1488219.html
+load 1600367.html

--- a/layout/forms/nsFieldSetFrame.cpp
+++ b/layout/forms/nsFieldSetFrame.cpp
@@ -735,16 +735,17 @@ void nsFieldSetFrame::Reflow(nsPresConte
             ? aReflowInput.ApplyMinMaxBSize(0)
             : aReflowInput.ComputedBSize();
     finalSize.BSize(wm) =
         contentBoxBSize +
         aReflowInput.ComputedLogicalBorderPadding().BStartEnd(wm);
   }
 
   if (aStatus.IsComplete() &&
+      aReflowInput.AvailableBSize() != NS_UNCONSTRAINEDSIZE &&
       finalSize.BSize(wm) > aReflowInput.AvailableBSize() &&
       border.BEnd(wm) > 0 && aReflowInput.AvailableBSize() > border.BEnd(wm)) {
     // Our end border doesn't fit but it should fit in the next column/page.
     if (MOZ_UNLIKELY(avoidBreakInside)) {
       aDesiredSize.SetSize(wm, LogicalSize(wm));
       aStatus.SetInlineLineBreakBeforeAndReset();
       return;
     } else {
@@ -752,16 +753,18 @@ void nsFieldSetFrame::Reflow(nsPresConte
           StyleBoxDecorationBreak::Slice) {
         finalSize.BSize(wm) -= border.BEnd(wm);
       }
       aStatus.SetIncomplete();
     }
   }
 
   if (!aStatus.IsComplete()) {
+    MOZ_ASSERT(aReflowInput.AvailableBSize() != NS_UNCONSTRAINEDSIZE,
+               "must be Complete in an unconstrained available block-size");
     // Stretch our BSize to fill the fragmentainer.
     finalSize.BSize(wm) =
         std::max(finalSize.BSize(wm), aReflowInput.AvailableBSize());
   }
   aDesiredSize.SetSize(wm, finalSize);
   aDesiredSize.SetOverflowAreasToDesiredBounds();
 
   if (legend) {
@@ -825,16 +828,19 @@ nscoord nsFieldSetFrame::GetLogicalBasel
 bool nsFieldSetFrame::GetVerticalAlignBaseline(WritingMode aWM,
                                                nscoord* aBaseline) const {
   if (StyleDisplay()->IsContainLayout()) {
     // If we are layout-contained, our child 'inner' should not
     // affect how we calculate our baseline.
     return false;
   }
   nsIFrame* inner = GetInner();
+  if (MOZ_UNLIKELY(!inner)) {
+    return false;
+  }
   MOZ_ASSERT(!inner->GetWritingMode().IsOrthogonalTo(aWM));
   if (!inner->GetVerticalAlignBaseline(aWM, aBaseline)) {
     return false;
   }
   nscoord innerBStart = inner->BStart(aWM, GetSize());
   *aBaseline += innerBStart;
   return true;
 }
@@ -843,16 +849,19 @@ bool nsFieldSetFrame::GetNaturalBaseline
     WritingMode aWM, BaselineSharingGroup aBaselineGroup,
     nscoord* aBaseline) const {
   if (StyleDisplay()->IsContainLayout()) {
     // If we are layout-contained, our child 'inner' should not
     // affect how we calculate our baseline.
     return false;
   }
   nsIFrame* inner = GetInner();
+  if (MOZ_UNLIKELY(!inner)) {
+    return false;
+  }
   MOZ_ASSERT(!inner->GetWritingMode().IsOrthogonalTo(aWM));
   if (!inner->GetNaturalBaselineBOffset(aWM, aBaselineGroup, aBaseline)) {
     return false;
   }
   nscoord innerBStart = inner->BStart(aWM, GetSize());
   if (aBaselineGroup == BaselineSharingGroup::First) {
     *aBaseline += innerBStart;
   } else {