diff --git a/src/resources.py b/src/resources.py
index 2069598..1fc24d7 100644
--- a/src/resources.py
+++ b/src/resources.py
@@ -176,7 +176,7 @@ class SampleResource(Resource):
                 )
 
             try:
-                before = datetime.fromisoformat(request.args.get('after'))
+                before = datetime.fromisoformat(request.args.get('before'))
             except (ValueError, TypeError):
                 before = None
             else: