diff --git a/core/core.js b/core/core.js
index 7c39c1baed..f94ec8139c 100644
--- a/core/core.js
+++ b/core/core.js
@@ -44,18 +44,23 @@
   }
 
   function processResponse(res) {
-    // const [ok, err] = res;
-    if (res[1] === null) {
-      return res[0];
+    if (!isErr(res)) {
+      return res;
     }
-    throw processErr(res[1]);
+    throw processErr(res);
+  }
+
+  // .$err_class_name is a special key that should only exist on errors
+  function isErr(res) {
+    return !!(res && res.$err_class_name);
   }
 
   function processErr(err) {
-    const [ErrorClass, args] = getErrorClassAndArgs(err.className);
+    const className = err.$err_class_name;
+    const [ErrorClass, args] = getErrorClassAndArgs(className);
     if (!ErrorClass) {
       return new Error(
-        `Unregistered error class: "${err.className}"\n  ${err.message}\n  Classes of errors returned from ops should be registered via Deno.core.registerErrorClass().`,
+        `Unregistered error class: "${className}"\n  ${err.message}\n  Classes of errors returned from ops should be registered via Deno.core.registerErrorClass().`,
       );
     }
     return new ErrorClass(err.message, ...args);
@@ -82,13 +87,12 @@
   }
 
   function opAsyncHandler(promiseId, res) {
-    // const [ok, err] = res;
     const promise = promiseTable.get(promiseId);
     promiseTable.delete(promiseId);
-    if (!res[1]) {
-      promise.resolve(res[0]);
+    if (!isErr(res)) {
+      promise.resolve(res);
     } else {
-      promise.reject(processErr(res[1]));
+      promise.reject(processErr(res));
     }
   }
 
diff --git a/core/ops.rs b/core/ops.rs
index 3af60d072c..867bb52105 100644
--- a/core/ops.rs
+++ b/core/ops.rs
@@ -74,11 +74,16 @@ pub enum Op {
 }
 
 #[derive(Serialize)]
-pub struct OpResult<R>(Option<R>, Option<OpError>);
+#[serde(untagged)]
+pub enum OpResult<R> {
+  Ok(R),
+  Err(OpError),
+}
 
 #[derive(Serialize)]
 #[serde(rename_all = "camelCase")]
 pub struct OpError {
+  #[serde(rename = "$err_class_name")]
   class_name: &'static str,
   message: String,
 }
@@ -88,14 +93,11 @@ pub fn serialize_op_result<R: Serialize + 'static>(
   state: Rc<RefCell<OpState>>,
 ) -> OpResponse {
   OpResponse::Value(Box::new(match result {
-    Ok(v) => OpResult::<R>(Some(v), None),
-    Err(err) => OpResult::<R>(
-      None,
-      Some(OpError {
-        class_name: (state.borrow().get_error_class_fn)(&err),
-        message: err.to_string(),
-      }),
-    ),
+    Ok(v) => OpResult::Ok(v),
+    Err(err) => OpResult::Err(OpError {
+      class_name: (state.borrow().get_error_class_fn)(&err),
+      message: err.to_string(),
+    }),
   }))
 }