diff --git a/src/usimp.ts b/src/usimp.ts
index 6444fda..b893887 100644
--- a/src/usimp.ts
+++ b/src/usimp.ts
@@ -328,7 +328,7 @@ export class Session {
                         'data': data
                     }));
 
-                    if (cb === null) {
+                    if (cb === undefined) {
                         const responseData = await response;
                         const endTime = performance.now();
                         responseData.duration = endTime - startTime;