Fix build failure

diff --git a/generator/nanopb_generator.py b/generator/nanopb_generator.py
index 6094403..2a9ae00 100755 (executable)
--- a/generator/nanopb_generator.py
+++ b/generator/nanopb_generator.py
@@ -520,7 +520,7 @@ class ExtensionRange(Field):
         return '    pb_extension_t *extensions;'
     
     def types(self):
-        return None
+        return ''
     
     def tags(self):
         return ''