fix

src/reflector.ts

@@ -2,12 +2,22 @@ import 'reflect-metadata';
 import { ArrayValue, GenericMap, Key, MapValue, StringDict } from './def';
 
 export class Reflector<M extends StringDict, AM extends StringDict> {
+  private isClass(target: any) {
+    if (typeof target !== 'function') {
+      return false;
+    }
+    // eslint-disable-next-line @typescript-eslint/ban-types
+    return (target as Function).toString().startsWith('class');
+  }
+
   get<K extends Key<GenericMap<M, AM>>>(
     metadataKey: K,
     instance: any,
     key?: string | symbol,
   ): MapValue<M, AM, K> {
-    const instanceClass = instance.constructor;
+    const instanceClass = this.isClass(instance)
+      ? instance
+      : instance.constructor;
     if (key) {
       return Reflect.getMetadata(metadataKey, instanceClass, key);
     } else {