diff --git a/dom/indexedDB/ActorsParent.cpp b/dom/indexedDB/ActorsParent.cpp
index 9203f9ee41..52f221d78a 100644
--- a/dom/indexedDB/ActorsParent.cpp
+++ b/dom/indexedDB/ActorsParent.cpp
@@ -26311,13 +26311,17 @@ ObjectStoreAddOrPutRequestOp::DoDatabaseWork(DatabaseConnection* aConnection)
       return rv;
     }
   } else {
-    nsCString flatCloneData;
-    flatCloneData.SetLength(cloneDataSize);
-    auto iter = cloneData.Start();
-    cloneData.ReadBytes(iter, flatCloneData.BeginWriting(), cloneDataSize);
+    AutoTArray<char, 4096> flatCloneData;  // 4096 from JSStructuredCloneData
+    if (!flatCloneData.SetLength(cloneDataSize, fallible)) {
+      return NS_ERROR_OUT_OF_MEMORY;
+    }
+    { // iter scope
+      auto iter = cloneData.Start();
+      MOZ_ALWAYS_TRUE(cloneData.ReadBytes(iter, flatCloneData.Elements(), cloneDataSize));
+    }
 
     // Compress the bytes before adding into the database.
-    const char* uncompressed = flatCloneData.BeginReading();
+    const char* uncompressed = flatCloneData.Elements();
     size_t uncompressedLength = cloneDataSize;
 
     size_t compressedLength = snappy::MaxCompressedLength(uncompressedLength);