diff --git a/Library/Homebrew/diagnostic.rb b/Library/Homebrew/diagnostic.rb
index e6fecb4787..36a0d83a7e 100644
--- a/Library/Homebrew/diagnostic.rb
+++ b/Library/Homebrew/diagnostic.rb
@@ -99,7 +99,7 @@ module Homebrew
       end
 
       def check_build_from_source
-        return unless ENV.key?("HOMEBREW_BUILD_FROM_SOURCE")
+        return unless ENV("HOMEBREW_BUILD_FROM_SOURCE")
 
         <<-EOS.undent
           You have HOMEBREW_BUILD_FROM_SOURCE set. This environment variable is