diff --git a/tasklib/serializing.py b/tasklib/serializing.py
index 7b7a46e..172d882 100644
--- a/tasklib/serializing.py
+++ b/tasklib/serializing.py
@@ -237,7 +237,10 @@ def datetime_normalizer(self, value):
                 # time zone at this point. Also None is a valid value too.
                 localized = value
         elif isinstance(value, str):
-            localized = self.backend.convert_datetime_string(value)
+            if value == '':
+                localized = value
+            else:
+                localized = self.backend.convert_datetime_string(value)
         else:
             raise ValueError("Provided value could not be converted to "
                              "datetime, its type is not supported: {}"