diff --git a/Library/Homebrew/build_environment.rb b/Library/Homebrew/build_environment.rb
index 7bff387b11..4f4812bcdf 100644
--- a/Library/Homebrew/build_environment.rb
+++ b/Library/Homebrew/build_environment.rb
@@ -25,7 +25,7 @@ class BuildEnvironment
   end
 
   def modify_build_environment(receiver)
-    receiver.instance_eval(&proc)
+    receiver.instance_eval(&proc) if self.proc
   end
 
   def marshal_dump