diff --git a/mysql-test/main/get_diagnostics.result b/mysql-test/main/get_diagnostics.result
index 090642923ce..d394d96eaff 100644
--- a/mysql-test/main/get_diagnostics.result
+++ b/mysql-test/main/get_diagnostics.result
@@ -1734,3 +1734,16 @@ SELECT @row_num;
 @row_num
 105
 DROP PROCEDURE resignal_syntax;
+#
+# MDEV-26842: ROW_NUMBER is not set and differs from the message upon
+# WARN_DATA_TRUNCATED produced by inplace ALTER
+#
+CREATE TABLE t1 (a VARCHAR(64)) ENGINE=InnoDB;
+INSERT INTO t1 VALUES ('foo'),(null);
+ALTER TABLE t1 ADD PRIMARY KEY(a), ALGORITHM=INPLACE;
+ERROR 01000: Data truncated for column 'a' at row 2
+GET DIAGNOSTICS CONDITION 1 @n= ROW_NUMBER, @m= MESSAGE_TEXT;
+SELECT @n, @m;
+@n	@m
+2	Data truncated for column 'a' at row 2
+DROP TABLE t1;
diff --git a/mysql-test/main/get_diagnostics.test b/mysql-test/main/get_diagnostics.test
index 09a7b3bd0c3..f4495059f9c 100644
--- a/mysql-test/main/get_diagnostics.test
+++ b/mysql-test/main/get_diagnostics.test
@@ -1,3 +1,5 @@
+--source include/have_innodb.inc
+
 SET NAMES utf8;
 
 --echo #
@@ -1614,3 +1616,19 @@ GET DIAGNOSTICS CONDITION 1 @row_num= ROW_NUMBER;
 SELECT @row_num;
 
 DROP PROCEDURE resignal_syntax;
+
+--echo #
+--echo # MDEV-26842: ROW_NUMBER is not set and differs from the message upon
+--echo # WARN_DATA_TRUNCATED produced by inplace ALTER
+--echo #
+
+CREATE TABLE t1 (a VARCHAR(64)) ENGINE=InnoDB;
+INSERT INTO t1 VALUES ('foo'),(null);
+
+--error WARN_DATA_TRUNCATED
+ALTER TABLE t1 ADD PRIMARY KEY(a), ALGORITHM=INPLACE;
+
+GET DIAGNOSTICS CONDITION 1 @n= ROW_NUMBER, @m= MESSAGE_TEXT;
+SELECT @n, @m;
+
+DROP TABLE t1;
diff --git a/sql/field.cc b/sql/field.cc
index 63d39c77d95..b72b0357c23 100644
--- a/sql/field.cc
+++ b/sql/field.cc
@@ -11101,6 +11101,14 @@ Field::set_warning(Sql_condition::enum_warning_level level, uint code,
     will have table == NULL.
   */
   THD *thd= get_thd();
+
+  /*
+    In INPLACE ALTER, server can't know which row has generated
+    the warning, so the value of current row is supplied by the engine.
+  */
+  if (current_row)
+    thd->get_stmt_da()->reset_current_row_for_warning(current_row);
+
   if (thd->count_cuted_fields > CHECK_FIELD_EXPRESSION)
   {
     thd->cuted_fields+= cut_increment;