diff --git a/src/cogs.h b/src/cogs.h
index 11d7fe1d..bfb918e5 100644
--- a/src/cogs.h
+++ b/src/cogs.h
@@ -246,9 +246,15 @@ MDBX_NOTHROW_PURE_FUNCTION static inline const page_t *data_page(const void *dat
 
 MDBX_NOTHROW_PURE_FUNCTION static inline meta_t *page_meta(page_t *mp) { return (meta_t *)page_data(mp); }
 
-MDBX_NOTHROW_PURE_FUNCTION static inline size_t page_numkeys(const page_t *mp) { return mp->lower >> 1; }
+MDBX_NOTHROW_PURE_FUNCTION static inline size_t page_numkeys(const page_t *mp) {
+  assert(mp->lower <= mp->upper);
+  return mp->lower >> 1;
+}
 
-MDBX_NOTHROW_PURE_FUNCTION static inline size_t page_room(const page_t *mp) { return mp->upper - mp->lower; }
+MDBX_NOTHROW_PURE_FUNCTION static inline size_t page_room(const page_t *mp) {
+  assert(mp->lower <= mp->upper);
+  return mp->upper - mp->lower;
+}
 
 MDBX_NOTHROW_PURE_FUNCTION static inline size_t page_space(const MDBX_env *env) {
   STATIC_ASSERT(PAGEHDRSZ % 2 == 0);
diff --git a/src/tree-ops.c b/src/tree-ops.c
index cb5c02b3..b4d76d04 100644
--- a/src/tree-ops.c
+++ b/src/tree-ops.c
@@ -1228,6 +1228,7 @@ int page_split(MDBX_cursor *mc, const MDBX_val *const newkey, MDBX_val *const ne
 
     /* root split? */
     prev_top += mc->top - top;
+    cASSERT(mn, prev_top <= mn->top && prev_top <= mc->top);
 
     /* Right page might now have changed parent.
      * Check if left page also changed parent. */