diff --git a/test/property/test_webhook_handler.rb b/test/property/test_webhook_handler.rb
index ac650a9d2aa88c31848c71581c96b06780566e70..5d99eaf9cf468c6adb08c57b41fab2c2e0eb8382 100644
--- a/test/property/test_webhook_handler.rb
+++ b/test/property/test_webhook_handler.rb
@@ -139,4 +139,27 @@ class WebhookPropertyTest < Minitest::Test
 		}
 	end
 	em :test_unknown_outbound_returns_200
+
+	def test_unknown_direction_returns_400_ok_except_when_message_failed
+		property_of {
+			Webhook
+				.new(REDIS)
+				.direction { "unknown" }
+				.type {
+					choose(*(Webhook::INBOUND_TYPES << Webhook::OUTBOUND_TYPES)
+							.reject { |ty|
+								ty == "message-failed"
+							}
+					)
+				}
+				.generate
+		}.check { |metadata, example|
+			result = invoke_webhook(example)
+			assert_equal [400, {}, "OK"], result
+			assert_empty written
+			entries = REDIS.stream_entries("messages").sync
+			assert_empty entries
+		}
+	end
+	em :test_unknown_direction_returns_400_ok_except_when_message_failed
 end