diff --git a/drop_service/views.py b/drop_service/views.py
index f718836..4c05264 100644
--- a/drop_service/views.py
+++ b/drop_service/views.py
@@ -1,5 +1,6 @@
 import datetime
 import json
+import logging
 import uuid
 from email.utils import formatdate
 from time import mktime
@@ -16,6 +17,8 @@
 from .notify import get_notificators
 from .util import CsrfExemptView, check_drop_id, set_last_modified, utc_timestamp
 
+logger = logging.getLogger(__name__)
+
 
 def error(msg, status=status.HTTP_400_BAD_REQUEST):
     return HttpResponse(json.dumps({'error': msg}), status=status)
@@ -37,7 +40,8 @@ def _get_drops(self, drop_id):
         try:
             have_since, since = self.get_if_modified_since()
         except ValueError as value_error:
-            return error(str(value_error)), None
+            logger.warning('Could not parse modified-since header pack: %s', value_error)
+            have_since, since = False, None
 
         if have_since:
             drops = drops.filter(created_at__gt=since)
@@ -88,7 +92,10 @@ def get_if_modified_since(self):
         if coarse_since and finest_since:
             raise ValueError('Specify only one of X-Qabel-New-Since, If-Modified-Since')
         if coarse_since:
-            return True, dateparser.parse(coarse_since)
+            since = dateparser.parse(coarse_since)
+            if not since:
+                raise ValueError('Unable to parse If-Modified-Since')
+            return True, since
         elif finest_since:
             return True, datetime.datetime.fromtimestamp(float(finest_since), datetime.timezone.utc)
         else:
diff --git a/tests/test_rest.py b/tests/test_rest.py
index f0ed549..5afa53c 100644
--- a/tests/test_rest.py
+++ b/tests/test_rest.py
@@ -57,7 +57,18 @@ def test_get_messages_posted_since(self):
                                 HTTP_IF_MODIFIED_SINCE=format_datetime(dt, usegmt=True))
 
         assert response.status_code == status.HTTP_200_OK
-        assert 'Hello World' in response.content.decode()
+        body = response.content.decode()
+        assert 'Hello World' in body
+        assert 'Bar' not in body
+
+    def test_get_messages_posted_since_invalid(self):
+        response = self.app.get('/abcdefghijklmnopqrstuvwxyzabcdefghijklmnopo',
+                                HTTP_IF_MODIFIED_SINCE='warghabl')
+
+        assert response.status_code == status.HTTP_200_OK
+        body = response.content.decode()
+        assert 'Hello World' in body
+        assert 'Bar' not in body
 
     def test_get_messages_posted_since_qabel(self):
         response = self.app.get('/abcdefghijklmnopqrstuvwxyzabcdefghijklmnopo',
@@ -68,6 +79,15 @@ def test_get_messages_posted_since_qabel(self):
         assert 'Hello World' in body
         assert 'Bar' not in body
 
+    def test_get_messages_posted_since_qabel_invalid(self):
+        response = self.app.get('/abcdefghijklmnopqrstuvwxyzabcdefghijklmnopo',
+                                HTTP_X_QABEL_NEW_SINCE='warghabl')
+
+        assert response.status_code == status.HTTP_200_OK
+        body = response.content.decode()
+        assert 'Hello World' in body
+        assert 'Bar' not in body
+
     def test_get_qabel_round_trip(self):
         response = self.app.get('/abcdefghijklmnopqrstuvwxyzabcdefghijklmnopo')
         assert response.status_code == status.HTTP_200_OK