diff --git a/click/_compat.py b/click/_compat.py
index 69198fff7..f914d38b4 100644
--- a/click/_compat.py
+++ b/click/_compat.py
@@ -11,7 +11,7 @@
 MSYS2 = sys.platform.startswith('win') and ('GCC' in sys.version)
 # Determine local App Engine environment, per Google's own suggestion
 APP_ENGINE = ('APPENGINE_RUNTIME' in os.environ and
-              'Development/' in os.environ['SERVER_SOFTWARE'])
+              'Development/' in os.environ.get('SERVER_SOFTWARE', ''))
 WIN = sys.platform.startswith('win') and not APP_ENGINE and not MSYS2
 DEFAULT_COLUMNS = 80