diff --git a/lib/webmock/http_lib_adapters/async_http_client_adapter.rb b/lib/webmock/http_lib_adapters/async_http_client_adapter.rb
index 0591bb78..dfedd0df 100644
--- a/lib/webmock/http_lib_adapters/async_http_client_adapter.rb
+++ b/lib/webmock/http_lib_adapters/async_http_client_adapter.rb
@@ -16,6 +16,17 @@ class AsyncHttpClientAdapter < HttpLibAdapter
         OriginalAsyncHttpClient = Async::HTTP::Client unless const_defined?(:OriginalAsyncHttpClient)
 
         class << self
+          # protocol-http1 0.40.0 removed Protocol::HTTP1::Reason in favour of
+          # Protocol::HTTP::Status. Resolved on first use, rather than at load,
+          # so it does not depend on which of the two is required by then.
+          def status_descriptions
+            @status_descriptions ||= if defined?(::Protocol::HTTP1::Reason)
+                                       ::Protocol::HTTP1::Reason::DESCRIPTIONS
+                                     else
+                                       ::Protocol::HTTP::Status::DESCRIPTIONS
+                                     end
+          end
+
           def enable!
             Async::HTTP.send(:remove_const, :Client)
             Async::HTTP.send(:const_set, :Client, Async::HTTP::WebMockClientWrapper)
@@ -112,7 +123,7 @@ def build_webmock_response(response)
           webmock_response = WebMock::Response.new
           webmock_response.status = [
             response.status,
-            ::Protocol::HTTP1::Reason::DESCRIPTIONS[response.status]
+            WebMock::HttpLibAdapters::AsyncHttpClientAdapter.status_descriptions[response.status]
           ]
           webmock_response.headers = build_webmock_response_headers(response)
           webmock_response.body = body
diff --git a/spec/acceptance/async_http_client/async_http_client_spec.rb b/spec/acceptance/async_http_client/async_http_client_spec.rb
index 038954a8..ede98c22 100644
--- a/spec/acceptance/async_http_client/async_http_client_spec.rb
+++ b/spec/acceptance/async_http_client/async_http_client_spec.rb
@@ -156,6 +156,28 @@
       expect(callback_invoked).to eq(true)
     end
 
+    context 'with a real response' do
+      # build_webmock_response is the only place the adapter reads a status
+      # description, and only real responses reach it.
+      let(:server_url) { "http://#{WebMockServer.instance.host_with_port}/" }
+
+      after do
+        WebMock.reset_callbacks
+        WebMock.disable_net_connect!
+      end
+
+      it 'builds a webmock response with a status message' do
+        WebMock.allow_net_connect!
+
+        recorded_response = nil
+        WebMock.after_request { |_request, response| recorded_response = response }
+
+        make_request(:get, server_url)
+
+        expect(recorded_response.status).to eq([200, 'OK'])
+      end
+    end
+
     context 'scheme and protocol' do
       let(:default_response_headers) { {} }
 
diff --git a/spec/acceptance/async_http_client/async_http_client_spec_helper.rb b/spec/acceptance/async_http_client/async_http_client_spec_helper.rb
index bed7b457..fd5cf32c 100644
--- a/spec/acceptance/async_http_client/async_http_client_spec_helper.rb
+++ b/spec/acceptance/async_http_client/async_http_client_spec_helper.rb
@@ -62,7 +62,7 @@ def build_hash_response(response)
     {
 
       status: response.status.to_s,
-      message: Protocol::HTTP1::Reason::DESCRIPTIONS[response.status],
+      message: WebMock::HttpLibAdapters::AsyncHttpClientAdapter.status_descriptions[response.status],
       headers: build_response_headers(response),
       body: response.read
     }