diff --git a/chrome/content/zotero/xpcom/data/item.js b/chrome/content/zotero/xpcom/data/item.js
index 5bfb1f8b31..d7e940257b 100644
--- a/chrome/content/zotero/xpcom/data/item.js
+++ b/chrome/content/zotero/xpcom/data/item.js
@@ -3819,12 +3819,18 @@ Zotero.Item.prototype.fromJSON = function (json) {
 		// Item fields
 		default:
 			let fieldID = Zotero.ItemFields.getID(field);
+			if (!fieldID) {
+				Zotero.logError("Discarding unknown JSON field " + field + " for item "
+					+ this.libraryKey);
+				continue;
+			}
 			isValidForType[field] = Zotero.ItemFields.isValidForType(
 				Zotero.ItemFields.getFieldIDFromTypeAndBase(itemTypeID, fieldID) || fieldID,
 				this.itemTypeID
 			);
 			if (!isValidForType[field]) {
-				Zotero.logError("Discarding unknown JSON field " + field + " for item " + this.libraryKey);
+				Zotero.logError("Discarding invalid field " + field + " for type " + itemTypeID
+					+ " for item " + this.libraryKey);
 				continue;
 			}
 			this.setField(field, json[field]);
diff --git a/test/tests/itemTest.js b/test/tests/itemTest.js
index 74b1346959..ae69311b08 100644
--- a/test/tests/itemTest.js
+++ b/test/tests/itemTest.js
@@ -966,6 +966,17 @@ describe("Zotero.Item", function () {
 	})
 
 	describe("#fromJSON()", function () {
+		it("should ignore unknown fields", function* () {
+			var json = {
+				itemType: "journalArticle",
+				title: "Test",
+				foo: "Invalid"
+			};
+			var item = new Zotero.Item;
+			item.fromJSON(json);
+			assert.equal(item.getField('title'), 'Test');
+		})
+		
 		it("should accept ISO 8601 dates", function* () {
 			var json = {
 				itemType: "journalArticle",