diff --git a/compose/config/config.py b/compose/config/config.py
index 4c9cf423b..003b2e2f4 100644
--- a/compose/config/config.py
+++ b/compose/config/config.py
@@ -886,6 +886,8 @@ def merge_service_dicts(base, override, version):
 
 
 def merge_unique_items_lists(base, override):
+    override = [str(o) for o in override]
+    base = [str(b) for b in base]
     return sorted(set().union(base, override))
 
 
diff --git a/tests/unit/config/config_test.py b/tests/unit/config/config_test.py
index 9303abaf1..aaa7fbf8d 100644
--- a/tests/unit/config/config_test.py
+++ b/tests/unit/config/config_test.py
@@ -1486,6 +1486,44 @@ class ConfigTest(unittest.TestCase):
             'extends': {'service': 'foo'}
         }
 
+    def test_merge_service_dicts_heterogeneous(self):
+        base = {
+            'volumes': ['.:/app'],
+            'ports': ['5432']
+        }
+        override = {
+            'image': 'alpine:edge',
+            'ports': [5432]
+        }
+        actual = config.merge_service_dicts_from_files(
+            base,
+            override,
+            DEFAULT_VERSION)
+        assert actual == {
+            'image': 'alpine:edge',
+            'volumes': ['.:/app'],
+            'ports': ['5432']
+        }
+
+    def test_merge_service_dicts_heterogeneous_2(self):
+        base = {
+            'volumes': ['.:/app'],
+            'ports': [5432]
+        }
+        override = {
+            'image': 'alpine:edge',
+            'ports': ['5432']
+        }
+        actual = config.merge_service_dicts_from_files(
+            base,
+            override,
+            DEFAULT_VERSION)
+        assert actual == {
+            'image': 'alpine:edge',
+            'volumes': ['.:/app'],
+            'ports': ['5432']
+        }
+
     def test_merge_build_args(self):
         base = {
             'build': {