diff --git a/rust/ares_pma/c-src/btest-overrides.h b/rust/ares_pma/c-src/btest-overrides.h
index 94cee5c..4199461 100644
--- a/rust/ares_pma/c-src/btest-overrides.h
+++ b/rust/ares_pma/c-src/btest-overrides.h
@@ -10,4 +10,9 @@
 #undef BT_DAT_MAXKEYS
 #define BT_DAT_MAXKEYS 10
 
+/* maxdepth expanded because when BT_DAT_MAXKEYS is shrunk, the depth of the
+   btree can grow higher than usual */
+#undef BT_MAXDEPTH
+#define BT_MAXDEPTH 10
+
 #endif
diff --git a/rust/ares_pma/c-src/btree.c b/rust/ares_pma/c-src/btree.c
index 64b2513..aa9811f 100644
--- a/rust/ares_pma/c-src/btree.c
+++ b/rust/ares_pma/c-src/btree.c
@@ -367,7 +367,10 @@ _bt_insertdat(vaof_t lo, vaof_t hi, pgno_t fo,
 static int _bt_flip_meta(BT_state *);
 
 
-#define BT_MAXDEPTH 4           /* ;;: todo derive it */
+/* TODO: derive BT_MAXDEPTH */
+#ifndef BT_MAXDEPTH
+#define BT_MAXDEPTH 4
+#endif
 typedef struct BT_findpath BT_findpath;
 struct BT_findpath {
   BT_page *path[BT_MAXDEPTH];