diff --git a/pxtsim/simdriver.ts b/pxtsim/simdriver.ts
index f3f07a48661e..a2946a98b870 100644
--- a/pxtsim/simdriver.ts
+++ b/pxtsim/simdriver.ts
@@ -160,14 +160,11 @@ namespace pxsim {
                 } else {
                     const simUrl = this.getSimUrl();
                     // Ensure we preserve upload target path (/app/<sha>---simulator)
-                    let simPath = simUrl.pathname.replace(/---?.*/, "");
-                    // Remove leading and trailing slashes
-                    simPath = simPath.replace(/^\/+|\/+$/, "");
+                    const simPath = simUrl.pathname.replace(/---?.*/, "");
                     // Construct the path. The "-" element delineates the extension key from the resource name
-                    let simxPath = [simPath, "simx", key, "-", simx.index].join("/");
-                    // Append a leading slash to `simxPath` unless it already has one (if `simPath` was an empty string then it will already have a leading slash)
-                    simxPath = simxPath.startsWith("/") ? simxPath : `/${simxPath}`;
-                    simx.url = new URL(simxPath, simUrl.origin).toString();
+                    const simxPath = [simPath, "simx", key, "-", simx.index].join("/");
+                    // Create the fully-qualified URL, preserving the origin by removing all leading slashes
+                    simx.url = new URL(simxPath.replace(/^\/+/, ""), simUrl.origin).toString();
                 }
 
                 // Add the origin to the allowed origins