diff --git a/mfbt/SegmentedVector.h b/mfbt/SegmentedVector.h
index 2b7a097726f4..05f1a4c53189 100644
--- a/mfbt/SegmentedVector.h
+++ b/mfbt/SegmentedVector.h
@@ -289,7 +289,11 @@ class SegmentedVector : private AllocPolicy {
     }
 
    public:
-    bool Done() const { return !mSegment; }
+    bool Done() const {
+      MOZ_ASSERT_IF(mSegment, mSegment->isInList());
+      MOZ_ASSERT_IF(mSegment, mIndex < mSegment->Length());
+      return !mSegment;
+    }
 
     T& Get() {
       MOZ_ASSERT(!Done());