diff --git a/mysql-test/main/change_master_default.result b/mysql-test/main/change_master_default.result
index 71d76cca8182a..865361e670c31 100644
--- a/mysql-test/main/change_master_default.result
+++ b/mysql-test/main/change_master_default.result
@@ -258,6 +258,11 @@ master_ssl_crlpath
 using_gtid	Slave_Pos
 master_retry_count	100000
 slave_heartbeat_period	60.000
+#
+# MDEV-40122: `+DEFAULT` is not a valid value for master_heartbeat_period
+#
+CHANGE MASTER TO master_heartbeat_period= +DEFAULT;
+ERROR 42000: You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'DEFAULT' at line 1
 # Clean-up
 DROP PROCEDURE show_defaultable_fields;
 RESET SLAVE 'unset' ALL;
diff --git a/mysql-test/main/change_master_default.test b/mysql-test/main/change_master_default.test
index 971013dc3feed..37c937533f443 100644
--- a/mysql-test/main/change_master_default.test
+++ b/mysql-test/main/change_master_default.test
@@ -138,6 +138,15 @@ FROM information_schema.slave_status ORDER BY connection_name;
 --query_vertical CALL show_defaultable_fields()
 
 
+--echo #
+--echo # MDEV-40122: `+DEFAULT` is not a valid value for master_heartbeat_period
+--echo #
+
+# The `+` prefix may only precede a numeric literal, not `DEFAULT`
+--error ER_PARSE_ERROR
+CHANGE MASTER TO master_heartbeat_period= +DEFAULT;
+
+
 --echo # Clean-up
 
 DROP PROCEDURE show_defaultable_fields;
diff --git a/sql/sql_yacc.yy b/sql/sql_yacc.yy
index 83f8edd72630a..ac0ecd9054082 100644
--- a/sql/sql_yacc.yy
+++ b/sql/sql_yacc.yy
@@ -2423,14 +2423,14 @@ master_def:
             { mi->master_ssl_crlpath= path; };
           }
 
-        | MASTER_HEARTBEAT_PERIOD_SYM '=' opt_plus num_or_default
+        | MASTER_HEARTBEAT_PERIOD_SYM '=' num_or_default
           {
-            if ($4)
+            if ($3)
             {
               uint32_t milliseconds;
               bool overprecise;
               auto decimal_buf= my_decimal(),
-                  *decimal= $4->val_decimal(&decimal_buf);
+                  *decimal= $3->val_decimal(&decimal_buf);
               DBUG_ASSERT(decimal);
               if (Master_info_file::Heartbeat_period_value::from_decimal(
                 milliseconds, *decimal, overprecise
@@ -2490,7 +2490,7 @@ master_use_gtid_enum:
         | DEFAULT         { $$= enum_master_use_gtid::DEFAULT; }
         ;
 num_or_default:
-          NUM_literal { DBUG_ASSERT($$); }
+          opt_plus NUM_literal { DBUG_ASSERT($$); }
         | DEFAULT { $$= nullptr; }
         ;