diff --git a/src/core.c b/src/core.c
index b7e00790..20028fef 100644
--- a/src/core.c
+++ b/src/core.c
@@ -9302,8 +9302,13 @@ int mdbx_txn_begin_ex(MDBX_env *env, MDBX_txn *parent, MDBX_txn_flags_t flags,
     /* Nested transactions: Max 1 child, write txns only, no writemap */
     rc = check_txn_rw(parent,
                       MDBX_TXN_RDONLY | MDBX_WRITEMAP | MDBX_TXN_BLOCKED);
-    if (unlikely(rc != MDBX_SUCCESS))
+    if (unlikely(rc != MDBX_SUCCESS)) {
+      if (rc == MDBX_BAD_TXN && (parent->mt_flags & (MDBX_TXN_RDONLY | MDBX_TXN_BLOCKED)) == 0) {
+        ERROR("%s mode is incompatible with nested transactions", "MDBX_WRITEMAP");
+        rc = MDBX_INCOMPATIBLE;
+      }
       return rc;
+    }
 
     if (env->me_options.spill_parent4child_denominator) {
       /* Spill dirty-pages of parent to provide dirtyroom for child txn */