diff --git a/src/api-txn.c b/src/api-txn.c
index 24598992..6831fd5d 100644
--- a/src/api-txn.c
+++ b/src/api-txn.c
@@ -199,8 +199,13 @@ int mdbx_txn_begin_ex(MDBX_env *env, MDBX_txn *parent, MDBX_txn_flags_t flags, M
   if (parent) {
     /* 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->flags & (MDBX_TXN_RDONLY | MDBX_TXN_BLOCKED)) == 0) {
+        ERROR("%s mode is incompatible with nested transactions", "MDBX_WRITEMAP");
+        rc = MDBX_INCOMPATIBLE;
+      }
       return LOG_IFERR(rc);
+    }
 
     if (env->options.spill_parent4child_denominator) {
       /* Spill dirty-pages of parent to provide dirtyroom for child txn */