diff --git a/Library/Homebrew/formula.rb b/Library/Homebrew/formula.rb
index 876ca764f5..f54935939f 100644
--- a/Library/Homebrew/formula.rb
+++ b/Library/Homebrew/formula.rb
@@ -1664,7 +1664,7 @@ class Formula
       end
 
       script_path.dirname.mkpath
-      script_path.write Utils.safe_popen_read({ "SHELL" => shell }, executable, cmd, shell_parameter)
+      script_path.write Utils.safe_popen_read({ "SHELL" => shell.to_s }, executable, cmd, shell_parameter)
     end
   end