diff --git a/Library/Contributions/cmds/brew-unpack.rb b/Library/Contributions/cmds/brew-unpack.rb
index cee1d0ba30f46a61ea876b4c489b53616d0e7e79..86bab4eb574b13fdb63f7ae3467504b381d60917 100755
--- a/Library/Contributions/cmds/brew-unpack.rb
+++ b/Library/Contributions/cmds/brew-unpack.rb
@@ -23,6 +23,9 @@ module ScriptDataReader
   end
 end
 
+# otherwise we may unpack bottles
+ENV['HOMEBREW_BUILD_FROM_SOURCE'] = '1'
+
 # Need to tweak the Formula class slightly so that patching is option and `DATA`
 # patches work correctly.
 class Formula