diff --git a/compose/config/fields_schema.json b/compose/config/fields_schema.json
index 66cb2b4146..e79026265c 100644
--- a/compose/config/fields_schema.json
+++ b/compose/config/fields_schema.json
@@ -39,7 +39,7 @@
             {
               "type": "object",
               "patternProperties": {
-                "^[a-zA-Z0-9_]+$": {
+                "^[^-]+$": {
                   "type": ["string", "number", "boolean"],
                   "format": "environment"
                 }
diff --git a/tests/unit/config_test.py b/tests/unit/config_test.py
index f246d9f665..ff80270e6d 100644
--- a/tests/unit/config_test.py
+++ b/tests/unit/config_test.py
@@ -287,6 +287,21 @@ class ConfigTest(unittest.TestCase):
         self.assertTrue(mock_logging.warn.called)
         self.assertTrue(expected_warning_msg in mock_logging.warn.call_args[0][0])
 
+    def test_config_invalid_environment_dict_key_raises_validation_error(self):
+        expected_error_msg = "Service 'web' configuration key 'environment' contains an invalid type"
+
+        with self.assertRaisesRegexp(ConfigurationError, expected_error_msg):
+            config.load(
+                config.ConfigDetails(
+                    {'web': {
+                        'image': 'busybox',
+                        'environment': {'---': 'nope'}
+                    }},
+                    'working_dir',
+                    'filename.yml'
+                )
+            )
+
 
 class InterpolationTest(unittest.TestCase):
     @mock.patch.dict(os.environ)