diff --git a/lib/levenshtein.rb b/lib/levenshtein.rb
index b9def8f..f2f120d 100644
--- a/lib/levenshtein.rb
+++ b/lib/levenshtein.rb
@@ -9,19 +9,21 @@ class << self
     candidates = ['.bundle', '.so', '.dylib', ''].map { |ext| library + ext }
     ffi_lib(candidates)
 
+    # Safe version of distance, checks that arguments are really strings.
     def distance(str1, str2)
       validate(str1)
       validate(str2)
       ffi_distance(str1, str2)
     end
 
+    # Unsafe version. Results in a segmentation fault if passed nils!
+    attach_function :ffi_distance, :levenshtein, [:string, :string], :int
+
     private
     def validate(arg)
       unless arg.kind_of?(String)
         raise TypeError, "wrong argument type #{arg.class} (expected String)"
       end
     end
-
-    attach_function :ffi_distance, :levenshtein, [:string, :string], :int
   end
 end