diff --git a/js/packages/binderhub-client/lib/index.js b/js/packages/binderhub-client/lib/index.js
index 19d239304..2cd2a7d21 100644
--- a/js/packages/binderhub-client/lib/index.js
+++ b/js/packages/binderhub-client/lib/index.js
@@ -25,11 +25,14 @@ export class BinderRepository {
    * Call the BinderHub API
    */
   fetch() {
-    let apiUrl = this.baseUrl + "build/" + this.providerSpec;
+    let apiUrl =  new URL(this.baseUrl + "build/" + this.providerSpec);
+    let params = new URLSearchParams();
     if (this.buildToken) {
-        apiUrl = apiUrl + `?build_token=${this.buildToken}`;
+        params.append('build_token', this.buildToken);
     }
 
+    apiUrl.search = params.toString();
+
     this.eventSource = new EventSource(apiUrl);
     this.eventSource.onerror = (err) => {
       console.error("Failed to construct event stream", err);