diff --git a/src/spectator/mocks/stub.cr b/src/spectator/mocks/stub.cr
index b0a5a26..ee7b6ee 100644
--- a/src/spectator/mocks/stub.cr
+++ b/src/spectator/mocks/stub.cr
@@ -13,7 +13,7 @@ module Spectator
     getter constraint : AbstractArguments?
 
     # Creates the base of the stub.
-    def initialize(@method : Symbol, @constraint : Arguments? = nil)
+    def initialize(@method : Symbol, @constraint : AbstractArguments? = nil)
     end
 
     # Checks if a method call should receive the response from this stub.
diff --git a/src/spectator/mocks/value_stub.cr b/src/spectator/mocks/value_stub.cr
index 9636a5b..1202248 100644
--- a/src/spectator/mocks/value_stub.cr
+++ b/src/spectator/mocks/value_stub.cr
@@ -8,7 +8,7 @@ module Spectator
     getter value : T
 
     # Creates the stub.
-    def initialize(method : Symbol, @value : T, constraint : Arguments? = nil)
+    def initialize(method : Symbol, @value : T, constraint : AbstractArguments? = nil)
       super(method, constraint)
     end
   end