diff --git a/run_project_tests.py b/run_project_tests.py
index f28744f89..2bc76601c 100755
--- a/run_project_tests.py
+++ b/run_project_tests.py
@@ -512,6 +512,21 @@ if __name__ == '__main__':
     options = parser.parse_args()
     setup_commands(options.backend)
 
+    # Appveyor sets the `platform` environment variable which completely messes
+    # up building with the vs2010 and vs2015 backends.
+    #
+    # Specifically, MSBuild reads the `platform` environment variable to set
+    # the configured value for the platform (Win32/x64/arm), which breaks x86
+    # builds.
+    #
+    # Appveyor setting this also breaks our 'native build arch' detection for
+    # Windows in environment.py:detect_windows_arch() by overwriting the value
+    # of `platform` set by vcvarsall.bat.
+    #
+    # While building for x86, `platform` should be unset.
+    if 'APPVEYOR' in os.environ and os.environ['arch'] == 'x86':
+        os.environ.pop('platform')
+
     script_dir = os.path.split(__file__)[0]
     if script_dir != '':
         os.chdir(script_dir)