diff --git a/Library/Homebrew/formulary.rb b/Library/Homebrew/formulary.rb
index b6ac6a98ce..60479a875f 100644
--- a/Library/Homebrew/formulary.rb
+++ b/Library/Homebrew/formulary.rb
@@ -56,6 +56,7 @@ class Formulary
       begin
         have_klass = Formulary.formula_class_defined? class_name
       rescue NameError => e
+        raise unless e.name.to_s == class_name
         raise FormulaUnavailableError, name, e.backtrace
       end