diff --git a/appveyor.yml b/appveyor.yml
index 0c693b7..3def5fb 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -17,7 +17,8 @@ install:
 
   # Upgrade to the latest version of pip to avoid it displaying warnings
   # about it being out of date.
-  - "python -m pip install --upgrade pip"
+  # Disabled because it broke PyInstaller installation on AppVeyor.
+  #- "python -m pip install --upgrade pip"
 
   # Install the build dependencies of the project. If some dependencies contain
   # compiled extensions and are not provided as pre-built wheel packages,