diff --git a/mysql-test/r/merge.result b/mysql-test/r/merge.result
index 7cb9e5bb423..f806829b969 100644
--- a/mysql-test/r/merge.result
+++ b/mysql-test/r/merge.result
@@ -3910,6 +3910,8 @@ CREATE TABLE m1(a INT) ENGINE = MERGE UNION (q1, q2);
 CREATE TRIGGER trg1 BEFORE DELETE ON t1
 FOR EACH ROW
 INSERT INTO m1 VALUES (1);
+DELETE FROM t1;
+ERROR HY000: Unable to open underlying table which is differently defined or of non-MyISAM type or doesn't exist
 DROP TRIGGER trg1;
 DROP TABLE t1;
 DROP TABLE m1;
diff --git a/mysql-test/t/merge.test b/mysql-test/t/merge.test
index 18ec2b3d62f..959c7258a53 100644
--- a/mysql-test/t/merge.test
+++ b/mysql-test/t/merge.test
@@ -2945,9 +2945,8 @@ CREATE TRIGGER trg1 BEFORE DELETE ON t1
 FOR EACH ROW
   INSERT INTO m1 VALUES (1);
 
-# Uncomment the following to lines when MDEV-5042 is fixed.
-#--error ER_WRONG_MRG_TABLE
-#DELETE FROM t1;
+--error ER_WRONG_MRG_TABLE
+DELETE FROM t1;
 
 DROP TRIGGER trg1;
 DROP TABLE t1;
diff --git a/storage/myisammrg/ha_myisammrg.cc b/storage/myisammrg/ha_myisammrg.cc
index bb225002dc0..a1fe8a6efb8 100644
--- a/storage/myisammrg/ha_myisammrg.cc
+++ b/storage/myisammrg/ha_myisammrg.cc
@@ -653,8 +653,12 @@ extern "C" MI_INFO *myisammrg_attach_children_callback(void *callback_param)
   {
     DBUG_PRINT("error", ("failed to open underlying table '%s'.'%s'",
                          child_l->db, child_l->table_name));
-    /* This should only happen inside of CHECK/REPAIR TABLE. */
-    DBUG_ASSERT(current_thd->open_options & HA_OPEN_FOR_REPAIR);
+    /*
+      This should only happen inside of CHECK/REPAIR TABLE or
+      for the tables added by the pre-locking code.
+    */
+    DBUG_ASSERT(current_thd->open_options & HA_OPEN_FOR_REPAIR ||
+                child_l->prelocking_placeholder);
     goto end;
   }