diff --git a/mysql-test/r/variables.result b/mysql-test/r/variables.result
index e36f4165f46..13d09e09783 100644
--- a/mysql-test/r/variables.result
+++ b/mysql-test/r/variables.result
@@ -381,3 +381,19 @@ set global myisam_max_sort_file_size=4294967296;
 show global variables like 'myisam_max_sort_file_size';
 Variable_name	Value
 myisam_max_sort_file_size	MAX_FILE_SIZE
+set @tstlw = @@log_warnings;
+show global variables like 'log_warnings';
+Variable_name	Value
+log_warnings	1
+set global log_warnings = 0;
+show global variables like 'log_warnings';
+Variable_name	Value
+log_warnings	0
+set global log_warnings = 42;
+show global variables like 'log_warnings';
+Variable_name	Value
+log_warnings	42
+set global log_warnings = @tstlw;
+show global variables like 'log_warnings';
+Variable_name	Value
+log_warnings	1
diff --git a/mysql-test/t/variables.test b/mysql-test/t/variables.test
index b9aa52ec627..03e4778d9d6 100644
--- a/mysql-test/t/variables.test
+++ b/mysql-test/t/variables.test
@@ -276,3 +276,13 @@ select @@session.key_buffer_size;
 set global myisam_max_sort_file_size=4294967296;
 --replace_result 4294967296 MAX_FILE_SIZE 2146435072 MAX_FILE_SIZE
 show global variables like 'myisam_max_sort_file_size';
+
+# BUG#5135: cannot turn on log_warnings with SET in 4.1 (and 4.0)
+set @tstlw = @@log_warnings;
+show global variables like 'log_warnings';
+set global log_warnings = 0;
+show global variables like 'log_warnings';
+set global log_warnings = 42;
+show global variables like 'log_warnings';
+set global log_warnings = @tstlw;
+show global variables like 'log_warnings';
diff --git a/sql/mysqld.cc b/sql/mysqld.cc
index e4d60fc9e7c..e20251adac4 100644
--- a/sql/mysqld.cc
+++ b/sql/mysqld.cc
@@ -3959,11 +3959,11 @@ replicating a LOAD DATA INFILE command",
    0, 0, 0, 0},
   {"log-warnings", 'W', "Log some not critical warnings to the log file",
    (gptr*) &global_system_variables.log_warnings,
-   (gptr*) &max_system_variables.log_warnings, 0, GET_ULONG, OPT_ARG, 1, 0, 0,
+   (gptr*) &max_system_variables.log_warnings, 0, GET_ULONG, OPT_ARG, 1, 0, ~0L,
    0, 0, 0},
   {"warnings", 'W', "Deprecated ; Use --log-warnings instead",
    (gptr*) &global_system_variables.log_warnings,
-   (gptr*) &max_system_variables.log_warnings, 0, GET_ULONG, OPT_ARG, 1, 0, 0,
+   (gptr*) &max_system_variables.log_warnings, 0, GET_ULONG, OPT_ARG, 1, 0, ~0L,
    0, 0, 0},
   { "back_log", OPT_BACK_LOG,
     "The number of outstanding connection requests MySQL can have. This comes into play when the main MySQL thread gets very many connection requests in a very short time.",