diff --git a/appveyor.yml b/appveyor.yml
index 2c7debeb..1f8d71f2 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -1,3 +1,5 @@
 build_script:
   - pip install .
-  - python ./run_tests.py
+  # the '-u' flag is required so the output is in the correct order. 
+  # See https://github.com/joerick/cibuildwheel/pull/24 for more info.
+  - python -u ./run_tests.py