diff --git a/Formula/f/fx.rb b/Formula/f/fx.rb
index 4b1aa3af607d8..f324052b7eb2d 100644
--- a/Formula/f/fx.rb
+++ b/Formula/f/fx.rb
@@ -23,6 +23,6 @@ def install
   end
 
   test do
-    assert_equal "42", pipe_output("#{bin}/fx .", 42).strip
+    assert_equal "42", pipe_output("#{bin}/fx .", "42").strip
   end
 end