diff --git a/appveyor.yml b/appveyor.yml
index e2c9d60eb2..7b4e243500 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -27,6 +27,9 @@ environment:
 
 install:
 - cmd: "%PYTHON%\\python.exe -m pip install -U build pip setuptools twine wheel"
+# Pywin32 appears to be necessary to ensure sys.platform is working correctly.
+- cmd: "%PYTHON%\\python.exe -m pip install pywin32"
+- cmd: "%PYTHON%\\python.exe %PYTHON%\\Scripts\\pywin32_postinstall.py -install"
 - ps: If ($isWindows) { .\config\appveyor\install.ps1 }
 - sh: config/appveyor/install.sh