diff --git a/compose/config/serialize.py b/compose/config/serialize.py
index 5b36124d0..e364117e7 100644
--- a/compose/config/serialize.py
+++ b/compose/config/serialize.py
@@ -111,9 +111,9 @@ def denormalize_service_dict(service_dict, version, image_digest=None):
             )
 
     if 'ports' in service_dict and version not in (V3_2,):
-        service_dict['ports'] = map(
-            lambda p: p.legacy_repr() if isinstance(p, types.ServicePort) else p,
-            service_dict['ports']
-        )
+        service_dict['ports'] = [
+            p.legacy_repr() if isinstance(p, types.ServicePort) else p
+            for p in service_dict['ports']
+        ]
 
     return service_dict
diff --git a/tests/unit/config/config_test.py b/tests/unit/config/config_test.py
index b7e4cc9bf..6bf4986ff 100644
--- a/tests/unit/config/config_test.py
+++ b/tests/unit/config/config_test.py
@@ -3837,3 +3837,15 @@ class SerializeTest(unittest.TestCase):
         serialized_service = serialized_config['services']['web']
         assert secret_sort(serialized_service['secrets']) == secret_sort(service_dict['secrets'])
         assert 'secrets' in serialized_config
+
+    def test_serialize_ports(self):
+        config_dict = config.Config(version='2.0', services=[
+            {
+                'ports': [types.ServicePort('80', '8080', None, None, None)],
+                'image': 'alpine',
+                'name': 'web'
+            }
+        ], volumes={}, networks={}, secrets={})
+
+        serialized_config = yaml.load(serialize_config(config_dict))
+        assert '8080:80/tcp' in serialized_config['services']['web']['ports']