diff --git a/flask/helpers.py b/flask/helpers.py
index c6c2cddc..2f446327 100644
--- a/flask/helpers.py
+++ b/flask/helpers.py
@@ -58,7 +58,7 @@ def get_debug_flag(default=None):
     val = os.environ.get('FLASK_DEBUG')
     if not val:
         return default
-    return val not in ('0', 'false', 'no')
+    return val.lower() not in ('0', 'false', 'no')
 
 
 def _endpoint_from_view_func(view_func):