diff --git a/src/spectator/mocks/double.cr b/src/spectator/mocks/double.cr
index d546b37..4af8cf1 100644
--- a/src/spectator/mocks/double.cr
+++ b/src/spectator/mocks/double.cr
@@ -4,7 +4,7 @@ require "./unexpected_message_error"
 
 module Spectator::Mocks
   abstract class Double
-    def initialize(@spectator_double_name : Symbol, @null = false)
+    def initialize(@spectator_double_name : String, @null = false)
     end
 
     private macro stub(definition, &block)