diff --git a/.github/workflows/test-python.yml b/.github/workflows/test-python.yml
index 2053699c..cae36928 100644
--- a/.github/workflows/test-python.yml
+++ b/.github/workflows/test-python.yml
@@ -47,7 +47,7 @@ jobs:
         uses: actions/checkout@v4
         with:
           repository: 'mongodb-forks/django'
-          ref: 'mongodb-5.0.x'
+          ref: 'mongodb-5.1.x'
           path: 'django_repo'
       - name: Install system packages for Django's Python test dependencies
         run: |
diff --git a/django_mongodb/__init__.py b/django_mongodb/__init__.py
index 7994999d..9fe04e62 100644
--- a/django_mongodb/__init__.py
+++ b/django_mongodb/__init__.py
@@ -1,4 +1,4 @@
-__version__ = "5.0a0"
+__version__ = "5.1a0"
 
 # Check Django compatibility before other imports which may fail if the
 # wrong version of Django is installed.
diff --git a/django_mongodb/aggregates.py b/django_mongodb/aggregates.py
index 35daab2c..d65e5520 100644
--- a/django_mongodb/aggregates.py
+++ b/django_mongodb/aggregates.py
@@ -1,7 +1,7 @@
 from copy import deepcopy
 
 from django.db.models.aggregates import Aggregate, Count, StdDev, Variance
-from django.db.models.expressions import Case, Value, When
+from django.db.models.expressions import Case, Col, Value, When
 from django.db.models.lookups import IsNull
 from django.db.models.sql.where import WhereNode
 
@@ -19,7 +19,7 @@ def aggregate(
     resolve_inner_expression=False,
     **extra_context,  # noqa: ARG001
 ):
-    if self.filter:
+    if self.filter and not isinstance(self.filter, Col):
         node = self.copy()
         node.filter = None
         source_expressions = node.get_source_expressions()
diff --git a/django_mongodb/features.py b/django_mongodb/features.py
index 2aa8217d..ca2fc5d7 100644
--- a/django_mongodb/features.py
+++ b/django_mongodb/features.py
@@ -26,6 +26,12 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     uses_savepoints = False
 
     _django_test_expected_failures = {
+        # $concat only supports strings, not int
+        "db_functions.text.test_concat.ConcatTests.test_concat_non_str",
+        # QuerySet.order_by() with annotation transform doesn't work:
+        # "Expression $mod takes exactly 2 arguments. 1 were passed in"
+        # https://github.com/django/django/commit/b0ad41198b3e333f57351e3fce5a1fb47f23f376
+        "aggregation.tests.AggregateTestCase.test_order_by_aggregate_transform",
         # Database defaults not supported: bson.errors.InvalidDocument:
         # cannot encode object: <django.db.models.expressions.DatabaseDefault
         "basic.tests.ModelInstanceCreationTests.test_save_primary_with_db_default",
@@ -51,7 +57,7 @@ class DatabaseFeatures(BaseDatabaseFeatures):
         # Length of null considered zero rather than null.
         "db_functions.text.test_length.LengthTests.test_basic",
         # range lookup includes incorrect values.
-        "expressions.tests.IterableLookupInnerExpressionsTests.test_expressions_in_lookups_join_choice",
+        "expressions.tests.IterableLookupInnerExpressionsTests.test_expressions_range_lookups_join_choice",
         # Unexpected alias_refcount in alias_map.
         "queries.tests.Queries1Tests.test_order_by_tables",
         # The $sum aggregation returns 0 instead of None for null.
@@ -117,6 +123,7 @@ def django_test_expected_failures(self):
         "QuerySet.prefetch_related() is not supported on MongoDB.": {
             "m2m_through_regress.test_multitable.MultiTableTests.test_m2m_prefetch_proxied",
             "m2m_through_regress.test_multitable.MultiTableTests.test_m2m_prefetch_reverse_proxied",
+            "many_to_many.tests.ManyToManyQueryTests.test_prefetch_related_no_queries_optimization_disabled",
             "many_to_many.tests.ManyToManyTests.test_add_after_prefetch",
             "many_to_many.tests.ManyToManyTests.test_add_then_remove_after_prefetch",
             "many_to_many.tests.ManyToManyTests.test_clear_after_prefetch",
@@ -178,6 +185,7 @@ def django_test_expected_failures(self):
             "expressions.tests.BasicExpressionsTests.test_case_in_filter_if_boolean_output_field",
             "expressions.tests.BasicExpressionsTests.test_exists_in_filter",
             "expressions.tests.BasicExpressionsTests.test_order_by_exists",
+            "expressions.tests.BasicExpressionsTests.test_slicing_of_outerref",
             "expressions.tests.BasicExpressionsTests.test_subquery",
             "expressions.tests.ExistsTests.test_filter_by_empty_exists",
             "expressions.tests.ExistsTests.test_negated_empty_exists",
@@ -298,6 +306,7 @@ def django_test_expected_failures(self):
             "queries.tests.EmptyQuerySetTests.test_values_subquery",
             "queries.tests.ExcludeTests.test_exclude_subquery",
             "queries.tests.NullInExcludeTest.test_null_in_exclude_qs",
+            "queries.tests.Queries1Tests.test_combining_does_not_mutate",
             "queries.tests.Queries1Tests.test_ticket9985",
             "queries.tests.Queries1Tests.test_ticket9997",
             "queries.tests.Queries1Tests.test_ticket10742",
@@ -414,7 +423,11 @@ def django_test_expected_failures(self):
             "aggregation.tests.AggregateTestCase.test_count_star",
             "delete.tests.DeletionTests.test_only_referenced_fields_selected",
             "lookup.tests.LookupTests.test_in_ignore_none",
+            "lookup.tests.LookupTests.test_lookup_direct_value_rhs_unwrapped",
             "lookup.tests.LookupTests.test_textfield_exact_null",
+            "many_to_many.tests.ManyToManyQueryTests.test_count_join_optimization_disabled",
+            "many_to_many.tests.ManyToManyQueryTests.test_exists_join_optimization_disabled",
+            "many_to_many.tests.ManyToManyTests.test_custom_default_manager_exists_count",
             "queries.tests.ExistsSql.test_exists",
             "queries.tests.Queries6Tests.test_col_alias_quoted",
         },
diff --git a/django_mongodb/query_utils.py b/django_mongodb/query_utils.py
index fe3fed9b..32115a19 100644
--- a/django_mongodb/query_utils.py
+++ b/django_mongodb/query_utils.py
@@ -12,6 +12,8 @@ def process_lhs(node, compiler, connection):
         # node is a Func or Expression, possibly with multiple source expressions.
         result = []
         for expr in node.get_source_expressions():
+            if expr is None:
+                continue
             try:
                 result.append(expr.as_mql(compiler, connection))
             except FullResultSet: