--- a/widget/ContentCache.cpp
+++ b/widget/ContentCache.cpp
@@ -947,17 +947,17 @@ ContentCacheInParent::GetUnionTextRects(
   // in most cases.
 
   if (!aOffset && aOffset != mSelection.mAnchor &&
       aOffset != mSelection.mFocus && !mTextRectArray.InRange(aOffset)) {
     // The first character rect isn't cached.
     return false;
   }
 
-  if ((aRoundToExistingOffset && mTextRectArray.IsValid()) ||
+  if ((aRoundToExistingOffset && mTextRectArray.HasRects()) ||
       mTextRectArray.IsOverlappingWith(aOffset, aLength)) {
     aUnionTextRect =
       mTextRectArray.GetUnionRectAsFarAsPossible(aOffset, aLength,
                                                  aRoundToExistingOffset);
   } else {
     aUnionTextRect.SetEmpty();
   }
 
@@ -1306,16 +1306,18 @@ ContentCache::TextRectArray::GetUnionRec
 }
 
 LayoutDeviceIntRect
 ContentCache::TextRectArray::GetUnionRectAsFarAsPossible(
                                uint32_t aOffset,
                                uint32_t aLength,
                                bool aRoundToExistingOffset) const
 {
+  MOZ_ASSERT(HasRects());
+
   LayoutDeviceIntRect rect;
   if (!aRoundToExistingOffset && !IsOverlappingWith(aOffset, aLength)) {
     return rect;
   }
   uint32_t startOffset = std::max(aOffset, mStart);
   if (aRoundToExistingOffset && startOffset >= EndOffset()) {
     startOffset = EndOffset() - 1;
   }

--- a/widget/ContentCache.h
+++ b/widget/ContentCache.h
@@ -202,16 +202,20 @@ protected:
     {
       if (mStart == UINT32_MAX) {
         return false;
       }
       CheckedInt<uint32_t> endOffset =
         CheckedInt<uint32_t>(mStart) + mRects.Length();
       return endOffset.isValid();
     }
+    bool HasRects() const
+    {
+      return IsValid() && !mRects.IsEmpty();
+    }
     uint32_t StartOffset() const
     {
       NS_ASSERTION(IsValid(),
                    "The caller should check if the caret is valid");
       return mStart;
     }
     uint32_t EndOffset() const
     {