diff --git a/chrome/content/zotero/elements/itemBox.js b/chrome/content/zotero/elements/itemBox.js
index dd7bc8dd6b..a4e8cb4422 100644
--- a/chrome/content/zotero/elements/itemBox.js
+++ b/chrome/content/zotero/elements/itemBox.js
@@ -2233,7 +2233,9 @@
 			if (back) {
 				Zotero.debug('Looking for previous tabindex before ' + tabindex, 4);
 				for (let i = tabbableFields.length - 1; i >= 0; i--) {
-					if (parseInt(tabbableFields[i].getAttribute('ztabindex')) < tabindex) {
+					let field = tabbableFields[i];
+					let tabIndexHere = parseInt(field.getAttribute('ztabindex'));
+					if (tabIndexHere !== -1 && tabIndexHere < tabindex) {
 						next = tabbableFields[i];
 						break;
 					}
@@ -2242,7 +2244,9 @@
 			else {
 				Zotero.debug('Looking for tabindex ' + tabindex, 4);
 				for (var pos = 0; pos < tabbableFields.length; pos++) {
-					if (parseInt(tabbableFields[pos].getAttribute('ztabindex')) >= tabindex) {
+					let field = tabbableFields[pos];
+					let tabIndexHere = parseInt(field.getAttribute('ztabindex'));
+					if (tabIndexHere !== -1 && tabIndexHere >= tabindex) {
 						next = tabbableFields[pos];
 						break;
 					}