diff --git a/Library/Homebrew/extend/os/linux/diagnostic.rb b/Library/Homebrew/extend/os/linux/diagnostic.rb
index c68dbcc802..f583b965ae 100644
--- a/Library/Homebrew/extend/os/linux/diagnostic.rb
+++ b/Library/Homebrew/extend/os/linux/diagnostic.rb
@@ -60,7 +60,7 @@ module Homebrew
       end
 
       def check_xdg_data_dirs
-        xdg_data_dirs = ENV["XDG_DATA_DIRS"]
+        xdg_data_dirs = ENV.fetch("XDG_DATA_DIRS", nil)
         return if xdg_data_dirs.blank? || xdg_data_dirs.split("/").include?(HOMEBREW_PREFIX/"share")
 
         <<~EOS