diff --git a/Library/Homebrew/formula.rb b/Library/Homebrew/formula.rb
index fd2ccb0a65..cf1ad0deb1 100644
--- a/Library/Homebrew/formula.rb
+++ b/Library/Homebrew/formula.rb
@@ -1659,7 +1659,7 @@ class Formula
       elsif shell_parameter_format == :arg
         "--shell=#{shell}"
       elsif shell_parameter_format == :none
-        nil
+        ""
       else
         "#{shell_parameter_format}#{shell}"
       end