--- a/netwerk/test/httpserver/httpd.js
+++ b/netwerk/test/httpserver/httpd.js
@@ -898,16 +898,20 @@ class NodeServer {
   // > "bla"
   // let result = await NodeServer.execute(`f("test")`);
   // > "test"
   static execute(command) {
     let env = Cc["@mozilla.org/process/environment;1"].getService(
       Ci.nsIEnvironment
     );
     let h2Port = env.get("MOZNODE_EXEC_PORT");
+    if (!h2Port) {
+      throw new Error("Could not find MOZNODE_EXEC_PORT");
+    }
+
     let req = new XMLHttpRequest();
     req.open("POST", `http://127.0.0.1:${h2Port}/execute`);
 
     // Passing a function to NodeServer.execute will define that function
     // in node. It can be called in a later execute command.
     let isFunction = function(obj) {
       return !!(obj && obj.constructor && obj.call && obj.apply);
     };