diff --git a/chrome/content/zotero/xpcom/sync/syncEngine.js b/chrome/content/zotero/xpcom/sync/syncEngine.js
index 7eb6d2e3d..d3cb4d217 100644
--- a/chrome/content/zotero/xpcom/sync/syncEngine.js
+++ b/chrome/content/zotero/xpcom/sync/syncEngine.js
@@ -1000,7 +1000,7 @@ Zotero.Sync.Data.Engine.prototype._uploadObjects = Zotero.Promise.coroutine(func
 						}
 					);
 				}
-				batch.push(o.json);
+				batch.push(o);
 			}
 			
 			// No more non-failed requests
@@ -1011,8 +1011,10 @@ Zotero.Sync.Data.Engine.prototype._uploadObjects = Zotero.Promise.coroutine(func
 			// Remove selected and skipped objects from queue
 			queue.splice(0, batch.length + numSkipped);
 			
+			let jsonBatch = batch.map(o => o.json);
+			
 			Zotero.debug("UPLOAD BATCH:");
-			Zotero.debug(batch);
+			Zotero.debug(jsonBatch);
 			
 			let results;
 			let numSuccessful = 0;
@@ -1022,7 +1024,7 @@ Zotero.Sync.Data.Engine.prototype._uploadObjects = Zotero.Promise.coroutine(func
 				"POST",
 				libraryVersion,
 				objectType,
-				batch
+				jsonBatch
 			));
 			
 			// Mark successful and unchanged objects as synced with new version,
@@ -1038,8 +1040,8 @@ Zotero.Sync.Data.Engine.prototype._uploadObjects = Zotero.Promise.coroutine(func
 					let key = state == 'successful' ? current.key : current;
 					let changed = changedObjects.has(key);
 					
-					if (key != batch[index].key) {
-						throw new Error("Key mismatch (" + key + " != " + batch[index].key + ")");
+					if (key != jsonBatch[index].key) {
+						throw new Error("Key mismatch (" + key + " != " + jsonBatch[index].key + ")");
 					}
 					
 					let obj = objectsClass.getByLibraryAndKey(this.libraryID, key);
@@ -1070,13 +1072,14 @@ Zotero.Sync.Data.Engine.prototype._uploadObjects = Zotero.Promise.coroutine(func
 						// will guarantee that the item won't be redownloaded unnecessarily in the case of
 						// a full sync, because the version will be higher than whatever version is on the
 						// server.
-						batch[index].version = libraryVersion;
-						toCache.push(batch[index]);
+						jsonBatch[index].version = libraryVersion;
+						toCache.push(jsonBatch[index]);
 					}
 					
 					numSuccessful++;
 					// Remove from batch to mark as successful
 					delete batch[index];
+					delete jsonBatch[index];
 				}
 			}
 			yield Zotero.Sync.Data.Local.saveCacheObjects(
@@ -1107,7 +1110,7 @@ Zotero.Sync.Data.Engine.prototype._uploadObjects = Zotero.Promise.coroutine(func
 				if (data) {
 					e.data = data;
 				}
-				Zotero.logError("Error for " + objectType + " " + batch[index].key + " in "
+				Zotero.logError("Error for " + objectType + " " + jsonBatch[index].key + " in "
 					+ this.library.name + ":\n\n" + e);
 				
 				// This shouldn't happen, because the upload request includes a library version and should