date - MySQL weird date_add interval -

i want add 1 month on datetime on mysql 5.1.66-0+squeeze1.

when :

select date_add(now(), interval 1 month) 

result :

2013-08-19 17:36:34 

it's ok. when :

select date_add("2013-07-19 17:37:00", interval 1 month) 

result :

323031332d30382d31392031373a33373a3030 

same :

select date_add("2013-07-19", interval 1 month) 

result :

323031332d30382d3139 

i don't understand what's wrong :(

thanks.

edit : i've tried error same.

select date_add("2013-07-19", interval 31 day) select adddate("2013-07-19", 31) 

your exmple works fine me too, can configuration error... try casting result date:

select date(date_add('2013-07-19', interval 1 month))