diff --git a/mfbt/BufferList.h b/mfbt/BufferList.h
index 51eb966707c0..507aa62511c3 100644
--- a/mfbt/BufferList.h
+++ b/mfbt/BufferList.h
@@ -658,6 +658,20 @@ BufferList<AllocPolicy> BufferList<AllocPolicy>::Extract(IterImpl& aIter,
     aIter.mAbsoluteOffset -= removedBytes;
     mSize -= removedBytes;
 
+    // If our iterator is already at the end, we just removed the very last
+    // segment of our buffer list and need to shift the iterator back to point
+    // at the end of the previous segment.
+    if (aIter.Done()) {
+      MOZ_ASSERT(lastSegmentSize.isNothing());
+      if (mSegments.empty()) {
+        MOZ_ASSERT(aIter.mSegment == 0);
+        aIter.mData = aIter.mDataEnd = nullptr;
+      } else {
+        MOZ_ASSERT(aIter.mSegment == mSegments.length() - 1);
+        aIter.mData = aIter.mDataEnd = mSegments.back().End();
+      }
+    }
+
     if (lastSegmentSize.isSome()) {
       // We called reserve() on result.mSegments so infallibleAppend is safe.
       result.mSegments.infallibleAppend(
@@ -673,6 +687,15 @@ BufferList<AllocPolicy> BufferList<AllocPolicy>::Extract(IterImpl& aIter,
   AssertConsistentSize();
   result.AssertConsistentSize();
 
+  // Ensure that the iterator is still valid when Extract returns.
+#ifdef DEBUG
+  if (!mSegments.empty()) {
+    auto& segment = mSegments[aIter.mSegment];
+    MOZ_ASSERT(segment.Start() <= aIter.mData);
+    MOZ_ASSERT(aIter.mDataEnd == segment.End());
+  }
+#endif
+
   *aSuccess = true;
   return result;
 }
diff --git a/mfbt/tests/TestBufferList.cpp b/mfbt/tests/TestBufferList.cpp
index e80448dc3ba0..9535c2245fbd 100644
--- a/mfbt/tests/TestBufferList.cpp
+++ b/mfbt/tests/TestBufferList.cpp
@@ -299,6 +299,8 @@ int main(void) {
   MOZ_RELEASE_ASSERT(success);
   MOZ_RELEASE_ASSERT(bl11.Size() == 16);
   MOZ_RELEASE_ASSERT(iter.Done());
+  MOZ_RELEASE_ASSERT(iter.AdvanceAcrossSegments(bl10, 0));
+  MOZ_RELEASE_ASSERT(iter.Done());
   iter = bl11.Iter();
   MOZ_RELEASE_ASSERT(bl11.ReadBytes(iter, data, 16));
   MOZ_RELEASE_ASSERT(memcmp(data, "abcdefgh12345678", 16) == 0);