diff --git a/django_mongodb/features.py b/django_mongodb/features.py
index d0e6aac45..2c3a00a98 100644
--- a/django_mongodb/features.py
+++ b/django_mongodb/features.py
@@ -103,14 +103,12 @@ class DatabaseFeatures(BaseDatabaseFeatures):
         "schema.tests.SchemaTests.test_text_field_with_db_index",
         # AlterField
         "schema.tests.SchemaTests.test_alter_field_add_index_to_integerfield",
-        "schema.tests.SchemaTests.test_alter_field_default_dropped",
         "schema.tests.SchemaTests.test_alter_field_fk_keeps_index",
         "schema.tests.SchemaTests.test_alter_field_fk_to_o2o",
         "schema.tests.SchemaTests.test_alter_field_o2o_keeps_unique",
         "schema.tests.SchemaTests.test_alter_field_o2o_to_fk",
         "schema.tests.SchemaTests.test_alter_int_pk_to_int_unique",
         "schema.tests.SchemaTests.test_alter_not_unique_field_to_primary_key",
-        "schema.tests.SchemaTests.test_alter_null_to_not_null",
         # AlterField (db_index)
         "schema.tests.SchemaTests.test_alter_renames_index",
         "schema.tests.SchemaTests.test_indexes",
diff --git a/django_mongodb/schema.py b/django_mongodb/schema.py
index d2f109c6d..4e0f3581b 100644
--- a/django_mongodb/schema.py
+++ b/django_mongodb/schema.py
@@ -38,11 +38,16 @@ def _alter_field(
         new_db_params,
         strict=False,
     ):
+        collection = self.connection.database[model._meta.db_table]
         # Have they renamed the column?
         if old_field.column != new_field.column:
-            self.connection.database[model._meta.db_table].update_many(
-                {}, {"$rename": {old_field.column: new_field.column}}
-            )
+            collection.update_many({}, {"$rename": {old_field.column: new_field.column}})
+        # Replace NULL with the field default if the field and was changed from
+        # NULL to NOT NULL.
+        if new_field.has_default() and old_field.null and not new_field.null:
+            column = new_field.column
+            default = self.effective_default(new_field)
+            collection.update_many({column: {"$eq": None}}, [{"$set": {column: default}}])
 
     def remove_field(self, model, field):
         # Remove implicit M2M tables.