diff --git a/packages/proto-loader/src/index.ts b/packages/proto-loader/src/index.ts
index 9b8704c2..a3554f75 100644
--- a/packages/proto-loader/src/index.ts
+++ b/packages/proto-loader/src/index.ts
@@ -269,7 +269,7 @@ export function load(filename: string, options?: Options): Promise<PackageDefini
   const root: Protobuf.Root = new Protobuf.Root();
   options = options || {};
   if (!!options.includeDirs) {
-    if (!(options.includeDirs instanceof Array)) {
+    if (!(Array.isArray(options.includeDirs))) {
       return Promise.reject(new Error('The includeDirs option must be an array'));
     }
     addIncludePathResolver(root, options.includeDirs as string[]);
@@ -284,7 +284,7 @@ export function loadSync(filename: string, options?: Options): PackageDefinition
   const root: Protobuf.Root = new Protobuf.Root();
   options = options || {};
   if (!!options.includeDirs) {
-    if (!(options.includeDirs instanceof Array)) {
+    if (!(Array.isArray(options.includeDirs))) {
       throw new Error('The include option must be an array');
     }
     addIncludePathResolver(root, options.includeDirs as string[]);