diff --git a/pyscript.core/esm/index.js b/pyscript.core/esm/index.js
index 2deb1d05..15276818 100644
--- a/pyscript.core/esm/index.js
+++ b/pyscript.core/esm/index.js
@@ -13,8 +13,18 @@ const RUNTIME_SELECTOR = selectors.join(",");
 
 // ensure both runtime and its queue are awaited then returns the runtime
 const awaitRuntime = async (key) => {
-    const { runtime, queue } = runtimes.get(key);
-    return (await all([runtime, queue]))[0];
+    if (runtimes.has(key)) {
+        const { runtime, queue } = runtimes.get(key);
+        return (await all([runtime, queue]))[0];
+    }
+
+    const available = runtimes.size
+        ? `Available runtimes are: ${[...runtimes.keys()]
+              .map((r) => `"${r}"`)
+              .join(", ")}.`
+        : `There are no runtimes in this page.`;
+
+    throw new Error(`The runtime "${key}" was not found. ${available}`);
 };
 
 defineProperty(globalThis, "pyscript", {
diff --git a/pyscript.core/package.json b/pyscript.core/package.json
index 5a49fc0c..bc5e57a2 100644
--- a/pyscript.core/package.json
+++ b/pyscript.core/package.json
@@ -50,6 +50,6 @@
         "coincident": "^0.2.3"
     },
     "worker": {
-        "blob": "sha256-T0DDfoQde+3gciIoHqPGP2SgMQpbY/nAMjfd1cg8jyA="
+        "blob": "sha256-XIl723wj4b0gyB5IGnLem05DRvl5bzkeMahwAmQFMrM="
     }
 }