diff --git a/Library/Homebrew/cask/artifact/abstract_artifact.rb b/Library/Homebrew/cask/artifact/abstract_artifact.rb
index 25b6f02833..047093c357 100644
--- a/Library/Homebrew/cask/artifact/abstract_artifact.rb
+++ b/Library/Homebrew/cask/artifact/abstract_artifact.rb
@@ -99,7 +99,12 @@ module Cask
         description = key ? "#{stanza} #{key.inspect}" : stanza.to_s
 
         # backward-compatible string value
-        arguments = { executable: arguments } if arguments.is_a?(String)
+        if arguments.is_a?(String)
+          arguments = { executable: arguments }
+        else
+          # Avoid mutating the original argument
+          arguments = arguments.dup
+        end
 
         # key sanity
         permitted_keys = [:args, :input, :executable, :must_succeed, :sudo, :print_stdout, :print_stderr]