diff --git a/Library/Homebrew/cask/url.rb b/Library/Homebrew/cask/url.rb
index fd4865f313..f2e88920c9 100644
--- a/Library/Homebrew/cask/url.rb
+++ b/Library/Homebrew/cask/url.rb
@@ -157,7 +157,7 @@ module Cask
         trust_cert:      T.nilable(T::Boolean),
         cookies:         T.nilable(T::Hash[String, String]),
         referer:         T.nilable(T.any(URI::Generic, String)),
-        header:          T.nilable(String),
+        header:          T.nilable(T.any(String, T::Array[String])),
         user_agent:      T.nilable(T.any(Symbol, String)),
         data:            T.nilable(T::Hash[String, String]),
         only_path:       T.nilable(String),