Two minor fixes

lib/spack/spack/compilers/__init__.py

@@ -74,7 +74,7 @@ def _to_dict(compiler):
     d['spec'] = str(compiler.spec)
     d['paths'] = dict( (attr, getattr(compiler, attr, None)) for attr in _path_instance_vars )
     d['operating_system'] = compiler.operating_system.to_dict()
-    d['modules'] = compiler.modules
+    d['modules'] = compiler.modules if compiler.modules else []
 
     if not compiler.alias:
         yaml_text = yaml.dump(

lib/spack/spack/yaml_version_check.py

@@ -46,7 +46,7 @@ def check_compiler_yaml_version():
         if data:
             compilers = data['compilers'].items()
             if len(compilers) > 0:
-                if 'operating_system' not in compilers[0][1]:
+                if (not isinstance(compilers, list)) or 'operating_system' not in compilers[0][1]:
                     new_file = os.path.join(scope.path, '_old_compilers.yaml')
                     tty.warn('%s in out of date compilers format. '
                              'Moved to %s. Spack automatically generate '