diff --git a/bin/make-sizes b/bin/make-sizes
index 8facb57..0afa3d3 100755
--- a/bin/make-sizes
+++ b/bin/make-sizes
@@ -21,7 +21,7 @@ unit sub MAIN(
   Bool :n(:$dry-run) = False,
   Bool :v(:$verbose) = False,
   #| out dir, default ./out
-  IO :o(:$outdir) = 'out'.IO,
+  IO(Str) :o(:$outdir) = 'out'.IO,
   #| defaults to all *.png & *.jpg files in the current dir
   *@files,
 );