--- a/toolkit/components/osfile/modules/osfile_async_worker.js
+++ b/toolkit/components/osfile/modules/osfile_async_worker.js
@@ -46,17 +46,17 @@ if (this.Components) {
    let durationMs;
    try {
      let method = data.fun;
      LOG("Calling method", method);
      result = Agent[method].apply(Agent, data.args);
      LOG("Method", method, "succeeded");
    } catch (ex) {
      exn = ex;
-     LOG("Error while calling agent method", exn, exn.stack);
+     LOG("Error while calling agent method", exn, exn.stack || "");
    }
 
    if (start) {
      // Record duration
      durationMs = Date.now() - start;
      LOG("Method took", durationMs, "ms");
    }