diff --git a/Library/Homebrew/exceptions.rb b/Library/Homebrew/exceptions.rb
index 431d89dfd5..a1fed4b862 100644
--- a/Library/Homebrew/exceptions.rb
+++ b/Library/Homebrew/exceptions.rb
@@ -173,6 +173,6 @@ class ChecksumMismatchError < RuntimeError
   end
 
   def to_s
-    super + advice
+    super + advice.to_s
   end
 end