diff --git a/chrome/content/zotero/xpcom/sync/syncAPIClient.js b/chrome/content/zotero/xpcom/sync/syncAPIClient.js
index 5a6fce1115..19cf644d05 100644
--- a/chrome/content/zotero/xpcom/sync/syncAPIClient.js
+++ b/chrome/content/zotero/xpcom/sync/syncAPIClient.js
@@ -820,7 +820,7 @@ Zotero.Sync.APIClient.prototype = {
 	
 	_checkBackoff: function (xmlhttp) {
 		var backoff = xmlhttp.getResponseHeader("Backoff");
-		if (backoff && Number.isInteger(backoff)) {
+		if (backoff && parseInt(backoff) == backoff) {
 			// TODO: Update status?
 			this.caller.pause(backoff * 1000);
 		}
@@ -831,7 +831,7 @@ Zotero.Sync.APIClient.prototype = {
 		var retryAfter = xmlhttp.getResponseHeader("Retry-After");
 		var delay;
 		if (!retryAfter) return false;
-		if (!Number.isInteger(retryAfter)) {
+		if (parseInt(retryAfter) != retryAfter) {
 			Zotero.logError(`Invalid Retry-After delay ${retryAfter}`);
 			return false;
 		}
diff --git a/test/tests/syncAPIClientTest.js b/test/tests/syncAPIClientTest.js
index ddbffa0416..f3a4e9d795 100644
--- a/test/tests/syncAPIClientTest.js
+++ b/test/tests/syncAPIClientTest.js
@@ -174,7 +174,7 @@ describe("Zotero.Sync.APIClient", function () {
 							req.respond(
 								503,
 								{
-									"Retry-After": 5
+									"Retry-After": "5"
 								},
 								""
 							);
@@ -183,7 +183,7 @@ describe("Zotero.Sync.APIClient", function () {
 							req.respond(
 								503,
 								{
-									"Retry-After": 10
+									"Retry-After": "10"
 								},
 								""
 							);