diff --git a/src/spectator/mocks/verifying_double.cr b/src/spectator/mocks/verifying_double.cr
index ecdb1e9..2c64b14 100644
--- a/src/spectator/mocks/verifying_double.cr
+++ b/src/spectator/mocks/verifying_double.cr
@@ -1,5 +1,5 @@
 module Spectator::Mocks
-  class VerifyingDouble(T)
+  abstract class VerifyingDouble(T)
     def initialize(@null = false)
     end