diff --git a/spec/middleware/filters_spec.cr b/spec/middleware/filters_spec.cr
index 599e7fe..b603868 100644
--- a/spec/middleware/filters_spec.cr
+++ b/spec/middleware/filters_spec.cr
@@ -186,5 +186,5 @@ describe "Kemal::Middleware::Filters" do
 end
 
 class FilterTest
-  property modified
+  property modified : String?
 end
diff --git a/src/kemal/config.cr b/src/kemal/config.cr
index b51322c..9ec539a 100644
--- a/src/kemal/config.cr
+++ b/src/kemal/config.cr
@@ -5,7 +5,7 @@ module Kemal
     @ssl : OpenSSL::SSL::Context
 
     property host_binding, ssl, port, env, public_folder, logging,
-      always_rescue, error_handler, serve_static, run
+      always_rescue, serve_static, run
 
     def initialize
       @host_binding = "0.0.0.0"
@@ -15,8 +15,8 @@ module Kemal
       @public_folder = "./public"
       @logging = true
       @logger = nil
+      @error_handler = nil
       @always_rescue = true
-      @error_handler = uninitialized Kemal::CommonExceptionHandler
       @run = false
       @ssl = uninitialized OpenSSL::SSL::Context
     end
@@ -62,7 +62,7 @@ module Kemal
 
     private def setup_error_handler
       if @always_rescue
-        @error_handler ||= Kemal::CommonExceptionHandler::INSTANCE
+        @error_handler ||= Kemal::CommonExceptionHandler.new
         HANDLERS.insert(1, @error_handler.not_nil!)
       end
     end
diff --git a/src/kemal/middleware/filters.cr b/src/kemal/middleware/filters.cr
index 2a2c177..b265ac0 100644
--- a/src/kemal/middleware/filters.cr
+++ b/src/kemal/middleware/filters.cr
@@ -23,7 +23,7 @@ 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 -> _)
+    def _add_route_filter(verb, path, type, &block : HTTP::Server::Context -> String)
       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)
@@ -33,12 +33,12 @@ 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 -> _)
+    def before(verb, path = "*", &block : HTTP::Server::Context -> String)
       _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 -> _)
+    def after(verb, path = "*", &block : HTTP::Server::Context -> String)
       _add_route_filter verb, path, :after, &block
     end
 
@@ -68,9 +68,9 @@ module Kemal::Middleware
   end
 
   class Block
-    property block : (HTTP::Server::Context -> ) 
+    property block : (HTTP::Server::Context -> String)
 
-    def initialize(&@block : HTTP::Server::Context -> _)
+    def initialize(&@block : HTTP::Server::Context -> String)
     end
 
     def call(context)
@@ -86,7 +86,7 @@ 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 -> _)
+    def {{type.id}}_{{method.id}}(path = "*", &block : HTTP::Server::Context -> String)
      Kemal::Middleware::Filter::INSTANCE.{{type.id}}({{method}}.upcase, path, &block)
     end
   {% end %}