diff --git a/compose/config.py b/compose/config.py
index 2427476f7..c054213eb 100644
--- a/compose/config.py
+++ b/compose/config.py
@@ -158,7 +158,7 @@ class ServiceLoader(object):
         if 'extends' not in service_dict:
             return service_dict
 
-        extends_options = validate_extends_options(service_dict['name'], service_dict['extends'])
+        extends_options = self.validate_extends_options(service_dict['name'], service_dict['extends'])
 
         if self.working_dir is None:
             raise Exception("No working_dir passed to ServiceLoader()")
@@ -194,25 +194,29 @@ class ServiceLoader(object):
     def signature(self, name):
         return (self.filename, name)
 
+    def validate_extends_options(self, service_name, extends_options):
+        error_prefix = "Invalid 'extends' configuration for %s:" % service_name
 
-def validate_extends_options(service_name, extends_options):
-    error_prefix = "Invalid 'extends' configuration for %s:" % service_name
+        if not isinstance(extends_options, dict):
+            raise ConfigurationError("%s must be a dictionary" % error_prefix)
 
-    if not isinstance(extends_options, dict):
-        raise ConfigurationError("%s must be a dictionary" % error_prefix)
-
-    if 'service' not in extends_options:
-        raise ConfigurationError(
-            "%s you need to specify a service, e.g. 'service: web'" % error_prefix
-        )
-
-    for k, _ in extends_options.items():
-        if k not in ['file', 'service']:
+        if 'service' not in extends_options:
             raise ConfigurationError(
-                "%s unsupported configuration option '%s'" % (error_prefix, k)
+                "%s you need to specify a service, e.g. 'service: web'" % error_prefix
             )
 
-    return extends_options
+        if 'file' not in extends_options and self.filename is None:
+            raise ConfigurationError(
+                "%s you need to specify a 'file', e.g. 'file: something.yml'" % error_prefix
+            )
+
+        for k, _ in extends_options.items():
+            if k not in ['file', 'service']:
+                raise ConfigurationError(
+                    "%s unsupported configuration option '%s'" % (error_prefix, k)
+                )
+
+        return extends_options
 
 
 def validate_extended_service_dict(service_dict, filename, service):
diff --git a/tests/unit/config_test.py b/tests/unit/config_test.py
index 48dd9afa8..4047a7253 100644
--- a/tests/unit/config_test.py
+++ b/tests/unit/config_test.py
@@ -459,6 +459,14 @@ class ExtendsTest(unittest.TestCase):
 
         self.assertRaisesRegexp(config.ConfigurationError, 'what', load_config)
 
+    def test_extends_validation_no_file_key_no_filename_set(self):
+        dictionary = {'extends': {'service': 'web'}}
+
+        def load_config():
+            return config.make_service_dict('myweb', dictionary, working_dir='tests/fixtures/extends')
+
+        self.assertRaisesRegexp(config.ConfigurationError, 'file', load_config)
+
     def test_extends_validation_valid_config(self):
         dictionary = {'extends': {'service': 'web', 'file': 'common.yml'}}