diff --git a/scripts/test-runner.js b/scripts/test-runner.js
index 796dfd781..efd80a1ae 100644
--- a/scripts/test-runner.js
+++ b/scripts/test-runner.js
@@ -507,9 +507,11 @@ function runTest(name, code, options: PrepackOptions, args) {
               if (code.includes(diagnosticExpectedComment)) {
                 let idx = code.indexOf(diagnosticExpectedComment);
                 let errorCodeString = code.substring(idx + diagnosticExpectedComment.length, code.indexOf("\n", idx));
-                let errorCodeSet = new Set();
-                expectedDiagnostics.set(severity, errorCodeSet);
-                errorCodeString.split(",").forEach(errorCode => errorCodeSet.add(errorCode.trim()));
+                if (errorCodeString.trim() !== "") {
+                  let errorCodeSet = new Set();
+                  expectedDiagnostics.set(severity, errorCodeSet);
+                  errorCodeString.split(",").forEach(errorCode => errorCodeSet.add(errorCode.trim()));
+                }
                 options.residual = false;
                 options.errorHandler = getErrorHandlerWithWarningCapture(diagnosticOutput, args.verbose);
               }
diff --git a/src/evaluators/CallExpression.js b/src/evaluators/CallExpression.js
index 1fd904dc5..c30ce9ebe 100644
--- a/src/evaluators/CallExpression.js
+++ b/src/evaluators/CallExpression.js
@@ -21,6 +21,7 @@ import {
   AbstractObjectValue,
   ConcreteValue,
   FunctionValue,
+  NativeFunctionValue,
   ObjectValue,
   Value,
 } from "../values/index.js";
@@ -266,7 +267,7 @@ function tryToEvaluateCallOrLeaveAsAbstract(
   let effects;
   let savedSuppressDiagnostics = realm.suppressDiagnostics;
   try {
-    realm.suppressDiagnostics = true;
+    realm.suppressDiagnostics = !(func instanceof NativeFunctionValue) || func.name !== "__optimize";
     effects = realm.evaluateForEffects(
       () => EvaluateDirectCall(realm, strictCode, env, ref, func, thisValue, ast.arguments, tailCall),
       undefined,
diff --git a/src/serializer/functions.js b/src/serializer/functions.js
index 25d967e7f..8bfb5ce97 100644
--- a/src/serializer/functions.js
+++ b/src/serializer/functions.js
@@ -370,7 +370,7 @@ export class Functions {
     };
     let oldReportPropertyAccess = this.realm.reportPropertyAccess;
     this.realm.reportPropertyAccess = (pb: PropertyBinding) => {
-      if (pb.object.refuseSerialization) return;
+      if (ObjectValue.refuseSerializationOnPropertyBinding(pb)) return;
       let location = this.realm.currentLocation;
       if (!location) return; // happens only when accessing an additional function property
       if (pbs.has(pb) && !conflicts.has(location)) reportConflict(location);
diff --git a/src/serializer/utils.js b/src/serializer/utils.js
index d58e1df77..9aca6a173 100644
--- a/src/serializer/utils.js
+++ b/src/serializer/utils.js
@@ -217,12 +217,14 @@ export function handleReportedSideEffect(
     exceptionHandler(`side-effects from mutating the binding ${name}${location}`);
   } else if (sideEffectType === "MODIFIED_PROPERTY" || sideEffectType === "MODIFIED_GLOBAL") {
     let name = "";
-    let key = ((binding: any): PropertyBinding).key;
+    let pb = ((binding: any): PropertyBinding);
+    let key = pb.key;
     if (typeof key === "string") {
       name = `"${key}"`;
     }
     if (sideEffectType === "MODIFIED_PROPERTY") {
-      exceptionHandler(`side-effects from mutating a property ${name}${location}`);
+      if (!ObjectValue.refuseSerializationOnPropertyBinding(pb))
+        exceptionHandler(`side-effects from mutating a property ${name}${location}`);
     } else {
       exceptionHandler(`side-effects from mutating the global object property ${name}${location}`);
     }
diff --git a/src/utils/generator.js b/src/utils/generator.js
index 22267c5ab..7ef59b182 100644
--- a/src/utils/generator.js
+++ b/src/utils/generator.js
@@ -559,7 +559,7 @@ export class Generator {
     for (let propertyBinding of modifiedProperties.keys()) {
       let object = propertyBinding.object;
       if (createdObjects.has(object)) continue; // Created Object's binding
-      if (object.refuseSerialization) continue; // modification to internal state
+      if (ObjectValue.refuseSerializationOnPropertyBinding(propertyBinding)) continue; // modification to internal state
       // modifications to intrinsic objects are tracked in the generator
       if (object.isIntrinsic()) continue;
       output.emitPropertyModification(propertyBinding);
diff --git a/src/values/ObjectValue.js b/src/values/ObjectValue.js
index 91b04de5c..6ac567983 100644
--- a/src/values/ObjectValue.js
+++ b/src/values/ObjectValue.js
@@ -1074,4 +1074,10 @@ export default class ObjectValue extends ConcreteValue {
   $OwnPropertyKeys(): Array<PropertyKeyValue> {
     return OrdinaryOwnPropertyKeys(this.$Realm, this);
   }
+
+  static refuseSerializationOnPropertyBinding(pb: PropertyBinding): boolean {
+    if (pb.object.refuseSerialization) return true;
+    if (pb.internalSlot && typeof pb.key === "string" && pb.key[0] === "_") return true;
+    return false;
+  }
 }
diff --git a/test/serializer/optimized-functions/Issue2262Regression.js b/test/serializer/optimized-functions/Issue2262Regression.js
new file mode 100644
index 000000000..c3c98a967
--- /dev/null
+++ b/test/serializer/optimized-functions/Issue2262Regression.js
@@ -0,0 +1,19 @@
+(function() {
+  let p = {};
+  function f(c) {
+    let o = {};
+    if (c) {
+      if (global.__makePartial) __makePartial(o);
+      throw o;
+    }
+  }
+  if (global.__optimize) __optimize(f);
+  inspect = function() {
+    try {
+      f(true);
+      return false;
+    } catch (e) {
+      return true;
+    }
+  };
+})();
diff --git a/test/serializer/optimized-functions/Issue2266Regression.js b/test/serializer/optimized-functions/Issue2266Regression.js
new file mode 100644
index 000000000..2b6666767
--- /dev/null
+++ b/test/serializer/optimized-functions/Issue2266Regression.js
@@ -0,0 +1,12 @@
+// expected Warning:
+function outer() {
+  function inner() {
+    return 42;
+  }
+  if (global.__optimize) __optimize(inner);
+  return inner;
+}
+if (global.__optimize) __optimize(outer);
+global.inspect = function() {
+  return outer()();
+};