diff --git a/appveyor.yml b/appveyor.yml
index 4bcd44c..c780bc0 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -35,7 +35,7 @@ install:
 
 test_script:
   - cd c:\projects\amphp
-  - phpdbg -qrr vendor/phpunit/phpunit/phpunit --colors=always --coverage-text --coverage-clover build/logs/clover.xml
+  - phpdbg -qrr vendor/phpunit/phpunit/phpunit --debug --colors=always --coverage-text --coverage-clover build/logs/clover.xml
   # Disable for now, because it can't be combined and files can't be shown on coveralls.io
   # https://github.com/php-coveralls/php-coveralls/issues/234  
   # - vendor/bin/coveralls -v