diff --git a/Library/Homebrew/os/mac/version.rb b/Library/Homebrew/os/mac/version.rb
index e19ecae70a..d40c94bcd7 100644
--- a/Library/Homebrew/os/mac/version.rb
+++ b/Library/Homebrew/os/mac/version.rb
@@ -23,7 +23,7 @@ module OS
 
       def <=>(other)
         @comparison_cache.fetch(other) do
-          v = SYMBOLS.fetch(other, other.to_s)
+          v = SYMBOLS.fetch(other) { other.to_s }
           @comparison_cache[other] = super(Version.new(v))
         end
       end