diff --git a/google/cloud/spanner_dbapi/cursor.py b/google/cloud/spanner_dbapi/cursor.py
index 254eb5734a..c2e893a098 100644
--- a/google/cloud/spanner_dbapi/cursor.py
+++ b/google/cloud/spanner_dbapi/cursor.py
@@ -99,7 +99,7 @@ def description(self):
                 # Size of the SQL type of the column.
                 display_size=code_to_display_size.get(field.type_.code),
                 # Client perceived size of the column.
-                internal_size=field.ByteSize(),
+                internal_size=field._pb.ByteSize(),
             )
             columns.append(column_info)