diff --git a/mysql-test/r/variables.result b/mysql-test/r/variables.result
index 19b48efe6b4..ad56ddc2e94 100644
--- a/mysql-test/r/variables.result
+++ b/mysql-test/r/variables.result
@@ -797,6 +797,9 @@ set @@query_prealloc_size = @test;
 select @@query_prealloc_size = @test;
 @@query_prealloc_size = @test
 1
+set global sql_mode=repeat('a',80);
+ERROR 42000: Variable 'sql_mode' can't be set to the value of 'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa'
+End of 4.1 tests
 create table t1 (a int);
 select a into @x from t1;
 Warnings:
diff --git a/mysql-test/t/variables.test b/mysql-test/t/variables.test
index 81db143b518..495cfe4445a 100644
--- a/mysql-test/t/variables.test
+++ b/mysql-test/t/variables.test
@@ -574,7 +574,14 @@ set @test = @@query_prealloc_size;
 set @@query_prealloc_size = @test;
 select @@query_prealloc_size = @test;
 
-# End of 4.1 tests
+#
+# Bug#31588 buffer overrun when setting variables
+#
+# Buffer-size Off By One. Should throw valgrind-warning without fix #31588.
+--error 1231
+set global sql_mode=repeat('a',80);
+
+--echo End of 4.1 tests
 
 #
 # Bug#6282 Packet error with SELECT INTO
diff --git a/sql/set_var.cc b/sql/set_var.cc
index 697de9cda97..65386e58202 100644
--- a/sql/set_var.cc
+++ b/sql/set_var.cc
@@ -1405,7 +1405,7 @@ bool sys_var::check_set(THD *thd, set_var *var, TYPELIB *enum_names)
 					    &not_used));
     if (error_len)
     {
-      strmake(buff, error, min(sizeof(buff), error_len));
+      strmake(buff, error, min(sizeof(buff) - 1, error_len));
       goto err;
     }
   }