diff --git a/Library/Homebrew/dev-cmd/tests.rb b/Library/Homebrew/dev-cmd/tests.rb
index bab63de71bcdd969d23d2637328a30a9e4fad42b..e687b0f6bc0340b34972b357e7f81b67cd289594 100644
--- a/Library/Homebrew/dev-cmd/tests.rb
+++ b/Library/Homebrew/dev-cmd/tests.rb
@@ -49,6 +49,8 @@ module Homebrew
       ENV["HOMEBREW_TEST_GENERIC_OS"] = "1" if args.generic?
       ENV["HOMEBREW_TEST_ONLINE"] = "1" if args.online?
 
+      ENV["USER"] ||= system_command!("id", args: ["-nu"]).stdout.chomp
+
       # Avoid local configuration messing with tests e.g. git being configured
       # to use GPG to sign by default
       ENV["HOME"] = "#{HOMEBREW_LIBRARY_PATH}/test"