diff --git a/src/spectator/mocks/arguments.cr b/src/spectator/mocks/arguments.cr
index a12aea9..96ae8f9 100644
--- a/src/spectator/mocks/arguments.cr
+++ b/src/spectator/mocks/arguments.cr
@@ -18,12 +18,12 @@ module Spectator
     end
 
     # Constructs an instance from literal arguments.
-    def self.capture(*args, **kwargs) : self
-      new(args, kwargs)
+    def self.capture(*args, **kwargs) : AbstractArguments
+      new(args, kwargs).as(AbstractArguments)
     end
 
     # Instance of empty arguments.
-    class_getter empty : Arguments(Tuple(), NamedTuple()) = capture
+    class_getter empty : AbstractArguments = capture
 
     # Returns the positional argument at the specified index.
     def [](index : Int)
diff --git a/src/spectator/mocks/method_call.cr b/src/spectator/mocks/method_call.cr
index 758a795..5604cad 100644
--- a/src/spectator/mocks/method_call.cr
+++ b/src/spectator/mocks/method_call.cr
@@ -11,12 +11,12 @@ module Spectator
     getter arguments : AbstractArguments
 
     # Creates a method call.
-    def initialize(@method : Symbol, @arguments : Arguments = Arguments.empty)
+    def initialize(@method : Symbol, @arguments : AbstractArguments = Arguments.empty)
     end
 
     # Creates a method call by splatting its arguments.
     def self.capture(method : Symbol, *args, **kwargs)
-      arguments = Arguments.new(args, kwargs)
+      arguments = Arguments.new(args, kwargs).as(AbstractArguments)
       new(method, arguments)
     end
 
diff --git a/src/spectator/mocks/proc_stub.cr b/src/spectator/mocks/proc_stub.cr
index 8cfa57e..d20cc9c 100644
--- a/src/spectator/mocks/proc_stub.cr
+++ b/src/spectator/mocks/proc_stub.cr
@@ -29,7 +29,7 @@ module Spectator
   module StubModifiers
     # Returns a new stub with an argument constraint.
     def with(*args, **kwargs, &block : AbstractArguments -> T) forall T
-      constraint = Arguments.new(args, kwargs)
+      constraint = Arguments.new(args, kwargs).as(AbstractArguments)
       ProcStub(T).new(method, block, constraint, location)
     end
   end