diff --git a/lib/Sources/ArgSource.ts b/lib/Sources/ArgSource.ts
index 141eb5d..e5ebc27 100644
--- a/lib/Sources/ArgSource.ts
+++ b/lib/Sources/ArgSource.ts
@@ -49,7 +49,7 @@ export class ArgSource extends OptionSource {
     }
 
     await Promise.all (options.map ((opt) => {
-      if (argv[opt.name] === 'undefined')
+      if (typeof argv[opt.name] === 'undefined')
         return Promise.resolve ();
       if (
         opt.type === 'array'