diff --git a/apiserver/database/fields.py b/apiserver/database/fields.py
index 52e7575..1d5e1c5 100644
--- a/apiserver/database/fields.py
+++ b/apiserver/database/fields.py
@@ -176,6 +176,13 @@ class SafeMapField(MapField, DictValidationMixin):
             self.error("Empty keys are not allowed in a MapField")
 
 
+class NullableStringField(StringField):
+    def validate(self, value):
+        if value is None:
+            return
+        super(NullableStringField, self).validate(value)
+
+
 class SafeDictField(DictField, DictValidationMixin):
     def validate(self, value):
         self._safe_validate(value)
diff --git a/apiserver/database/model/task/task.py b/apiserver/database/model/task/task.py
index 0837c01..a442ef7 100644
--- a/apiserver/database/model/task/task.py
+++ b/apiserver/database/model/task/task.py
@@ -18,6 +18,7 @@ from apiserver.database.fields import (
     UnionField,
     SafeSortedListField,
     EmbeddedDocumentListField,
+    NullableStringField,
 )
 from apiserver.database.model import AttributedDocument
 from apiserver.database.model.base import ProperDictMixin, GetMixin
@@ -260,7 +261,7 @@ class Task(AttributedDocument):
     configuration = SafeMapField(field=EmbeddedDocumentField(ConfigurationItem))
     runtime = SafeDictField(default=dict)
     models: Models = EmbeddedDocumentField(Models, default=Models)
-    container = SafeMapField(field=StringField(default=""))
+    container = SafeMapField(field=NullableStringField())
     enqueue_status = StringField(
         choices=get_options(TaskStatus), exclude_by_default=True
     )