diff --git a/marshal.c b/marshal.c
index d1218af9ad..ec38c2188a 100644
--- a/marshal.c
+++ b/marshal.c
@@ -173,7 +173,7 @@ struct dump_arg {
     st_table *data;
     st_table *compat_tbl;
     st_table *encodings;
-    unsigned long num_entries;
+    st_index_t num_entries;
 };
 
 struct dump_call_arg {