diff --git a/Library/Homebrew/cask/cask.rb b/Library/Homebrew/cask/cask.rb
index 2475b84da05da..caf666397c1a9 100644
--- a/Library/Homebrew/cask/cask.rb
+++ b/Library/Homebrew/cask/cask.rb
@@ -162,7 +162,7 @@ def hash
     end
 
     def eql?(other)
-      token == other.token
+      instance_of?(other.class) && token == other.token
     end
     alias == eql?