diff --git a/compose/config/fields_schema.json b/compose/config/fields_schema.json
index cc37f444..e254e353 100644
--- a/compose/config/fields_schema.json
+++ b/compose/config/fields_schema.json
@@ -41,7 +41,7 @@
               "type": "object",
               "patternProperties": {
                 "^[^-]+$": {
-                  "type": ["string", "number", "boolean"],
+                  "type": ["string", "number", "boolean", "null"],
                   "format": "environment"
                 }
               },
diff --git a/compose/config/validation.py b/compose/config/validation.py
index aea72286..8cfc405f 100644
--- a/compose/config/validation.py
+++ b/compose/config/validation.py
@@ -136,7 +136,7 @@ def process_errors(errors, service_name=None):
             if len(validator) >= 2:
                 first_type = anglicize_validator(validator[0])
                 last_type = anglicize_validator(validator[-1])
-                types_from_validator = "{}{}".format(first_type, ", ".join(validator[1:-1]))
+                types_from_validator = ", ".join([first_type] + validator[1:-1])
 
                 msg = "{} or {}".format(
                     types_from_validator,
@@ -156,7 +156,6 @@ def process_errors(errors, service_name=None):
         Inspecting the context value of a ValidationError gives us information about
         which sub schema failed and which kind of error it is.
         """
-
         required = [context for context in error.context if context.validator == 'required']
         if required:
             return required[0].message
diff --git a/tests/unit/config/config_test.py b/tests/unit/config/config_test.py
index e8caea81..e32e5b47 100644
--- a/tests/unit/config/config_test.py
+++ b/tests/unit/config/config_test.py
@@ -449,6 +449,23 @@ class InterpolationTest(unittest.TestCase):
         self.assertIn('in service "web"', cm.exception.msg)
         self.assertIn('"${"', cm.exception.msg)
 
+    def test_empty_environment_key_allowed(self):
+        service_dict = config.load(
+            build_config_details(
+                {
+                    'web': {
+                        'build': '.',
+                        'environment': {
+                            'POSTGRES_PASSWORD': ''
+                        },
+                    },
+                },
+                '.',
+                None,
+            )
+        )[0]
+        self.assertEquals(service_dict['environment']['POSTGRES_PASSWORD'], '')
+
 
 class VolumeConfigTest(unittest.TestCase):
     def test_no_binding(self):