diff --git a/src/kemal/middleware/filters.cr b/src/kemal/middleware/filters.cr
index c790c2c..c4c7491 100644
--- a/src/kemal/middleware/filters.cr
+++ b/src/kemal/middleware/filters.cr
@@ -26,7 +26,7 @@ module Kemal::Middleware
 
     # :nodoc: This shouldn't be called directly, it's not private because I need to call it for testing purpose since I can't call the macros in the spec.
     # It adds the block for the corresponding verb/path/type combination to the tree.
-    def _add_route_filter(verb, path, type, &block : HTTP::Server::Context -> String)
+    def _add_route_filter(verb, path, type, &block : HTTP::Server::Context -> _)
       lookup = lookup_filters_for_path_type(verb, path, type)
       if lookup.found? && lookup.payload.is_a?(Array(Block))
         (lookup.payload as Array(Block)) << Block.new(&block)
@@ -36,12 +36,12 @@ module Kemal::Middleware
     end
 
     # This can be called directly but it's simpler to just use the macros, it will check if another filter is not already defined for this verb/path/type and proceed to call `add_route_filter`
-    def before(verb, path = "*", &block : HTTP::Server::Context -> String)
+    def before(verb, path = "*", &block : HTTP::Server::Context -> _)
       _add_route_filter verb, path, :before, &block
     end
 
     # This can be called directly but it's simpler to just use the macros, it will check if another filter is not already defined for this verb/path/type and proceed to call `add_route_filter`
-    def after(verb, path = "*", &block : HTTP::Server::Context -> String)
+    def after(verb, path = "*", &block : HTTP::Server::Context -> _)
       _add_route_filter verb, path, :after, &block
     end
 
@@ -71,9 +71,10 @@ module Kemal::Middleware
   end
 
   class Block
-    property block : (HTTP::Server::Context -> String)
+    property block : HTTP::Server::Context -> String
 
-    def initialize(&@block : HTTP::Server::Context -> String)
+    def initialize(&block : HTTP::Server::Context -> _)
+      @block = ->(context : HTTP::Server::Context) { block.call(context).to_s}
     end
 
     def call(context)
@@ -89,7 +90,7 @@ end
 ALL_METHODS = %w(get post put patch delete all)
 {% for type in ["before", "after"] %}
   {% for method in ALL_METHODS %}
-    def {{type.id}}_{{method.id}}(path = "*", &block : HTTP::Server::Context -> String)
+    def {{type.id}}_{{method.id}}(path = "*", &block : HTTP::Server::Context -> _)
      Kemal::Middleware::Filter::INSTANCE.{{type.id}}({{method}}.upcase, path, &block)
     end
   {% end %}