diff --git a/python/descriptor_pool.c b/python/descriptor_pool.c
index e816f4e..45ea6a0 100644
--- a/python/descriptor_pool.c
+++ b/python/descriptor_pool.c
@@ -39,7 +39,7 @@
 typedef struct {
   PyObject_HEAD
   upb_symtab* symtab;
-  PyObject* db;
+  PyObject* db;  // The DescriptorDatabase underlying this pool.  May be NULL.
 } PyUpb_DescriptorPool;
 
 PyObject* PyUpb_DescriptorPool_GetDefaultPool() {