diff --git a/mysql-test/main/change_master_default.result b/mysql-test/main/change_master_default.result
index 71d76cca8182a..3bc203cc3a22c 100644
--- a/mysql-test/main/change_master_default.result
+++ b/mysql-test/main/change_master_default.result
@@ -258,6 +258,14 @@ 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
+CHANGE MASTER 'mdev40122' TO master_heartbeat_period= +45;
+CHANGE MASTER 'mdev40122' TO master_heartbeat_period= DEFAULT;
+RESET SLAVE 'mdev40122' ALL;
 # 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..6a2e5413351bc 100644
--- a/mysql-test/main/change_master_default.test
+++ b/mysql-test/main/change_master_default.test
@@ -138,6 +138,20 @@ 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;
+
+# `+` with a number and `DEFAULT` itself remain valid
+CHANGE MASTER 'mdev40122' TO master_heartbeat_period= +45;
+CHANGE MASTER 'mdev40122' TO master_heartbeat_period= DEFAULT;
+RESET SLAVE 'mdev40122' ALL;
+
+
 --echo # Clean-up
 
 DROP PROCEDURE show_defaultable_fields;
diff --git a/sql/sql_yacc.yy b/sql/sql_yacc.yy
index 83f8edd72630a..9d3cc35dda539 100644
--- a/sql/sql_yacc.yy
+++ b/sql/sql_yacc.yy
@@ -1628,7 +1628,6 @@ bool my_yyoverflow(short **a, YYSTYPE **b, size_t *yystacksize);
 
 %type <item_num>
         NUM_literal
-        num_or_default
 
 %type <item_basic_constant> text_literal
 
@@ -2423,34 +2422,33 @@ master_def:
             { mi->master_ssl_crlpath= path; };
           }
 
-        | MASTER_HEARTBEAT_PERIOD_SYM '=' opt_plus num_or_default
-          {
-            if ($4)
-            {
-              uint32_t milliseconds;
-              bool overprecise;
-              auto decimal_buf= my_decimal(),
-                  *decimal= $4->val_decimal(&decimal_buf);
-              DBUG_ASSERT(decimal);
-              if (Master_info_file::Heartbeat_period_value::from_decimal(
-                milliseconds, *decimal, overprecise
-              ))
-                my_yyabort_error((ER_SLAVE_HEARTBEAT_VALUE_OUT_OF_RANGE, MYF(0),
-                  Master_info_file::Heartbeat_period_value::MAX));
-              if (unlikely(milliseconds > slave_net_timeout*1000ULL))
-                push_warning(thd, Sql_condition::WARN_LEVEL_WARN,
-                  ER_SLAVE_HEARTBEAT_VALUE_OUT_OF_RANGE_MAX,
-                  ER_THD(thd, ER_SLAVE_HEARTBEAT_VALUE_OUT_OF_RANGE_MAX));
-              else if (unlikely(!milliseconds && overprecise))
-                push_warning(thd, Sql_condition::WARN_LEVEL_WARN,
-                  ER_SLAVE_HEARTBEAT_VALUE_OUT_OF_RANGE_MIN,
-                  ER_THD(thd, ER_SLAVE_HEARTBEAT_VALUE_OUT_OF_RANGE_MIN));
-              Lex->mi.heartbeat_period= [milliseconds](Master_info_file *mi)
-              { mi->master_heartbeat_period= milliseconds; };
-            }
-            else
-              Lex->mi.heartbeat_period= [](Master_info_file *mi)
-              { mi->master_heartbeat_period.set_default(); };
+        | MASTER_HEARTBEAT_PERIOD_SYM '=' opt_plus NUM_literal
+          {
+            uint32_t milliseconds;
+            bool overprecise;
+            auto decimal_buf= my_decimal(),
+                *decimal= $4->val_decimal(&decimal_buf);
+            DBUG_ASSERT(decimal);
+            if (Master_info_file::Heartbeat_period_value::from_decimal(
+              milliseconds, *decimal, overprecise
+            ))
+              my_yyabort_error((ER_SLAVE_HEARTBEAT_VALUE_OUT_OF_RANGE, MYF(0),
+                Master_info_file::Heartbeat_period_value::MAX));
+            if (unlikely(milliseconds > slave_net_timeout*1000ULL))
+              push_warning(thd, Sql_condition::WARN_LEVEL_WARN,
+                ER_SLAVE_HEARTBEAT_VALUE_OUT_OF_RANGE_MAX,
+                ER_THD(thd, ER_SLAVE_HEARTBEAT_VALUE_OUT_OF_RANGE_MAX));
+            else if (unlikely(!milliseconds && overprecise))
+              push_warning(thd, Sql_condition::WARN_LEVEL_WARN,
+                ER_SLAVE_HEARTBEAT_VALUE_OUT_OF_RANGE_MIN,
+                ER_THD(thd, ER_SLAVE_HEARTBEAT_VALUE_OUT_OF_RANGE_MIN));
+            Lex->mi.heartbeat_period= [milliseconds](Master_info_file *mi)
+            { mi->master_heartbeat_period= milliseconds; };
+          }
+        | MASTER_HEARTBEAT_PERIOD_SYM '=' DEFAULT
+          {
+            Lex->mi.heartbeat_period= [](Master_info_file *mi)
+            { mi->master_heartbeat_period.set_default(); };
           }
         | IGNORE_SERVER_IDS_SYM '=' '(' ignore_server_id_list ')'
           {
@@ -2487,12 +2485,8 @@ master_use_gtid_enum:
           NO_SYM          { $$= enum_master_use_gtid::NO; }
         | CURRENT_POS_SYM { $$= enum_master_use_gtid::CURRENT_POS; }
         | SLAVE_POS_SYM   { $$= enum_master_use_gtid::SLAVE_POS; }
-        | DEFAULT         { $$= enum_master_use_gtid::DEFAULT; }
-        ;
-num_or_default:
-          NUM_literal { DBUG_ASSERT($$); }
-        | DEFAULT { $$= nullptr; }
-        ;
+         | DEFAULT         { $$= enum_master_use_gtid::DEFAULT; }
+         ;
 
 ignore_server_id_list:
           /* Empty */