mysql_use_result returns NULL but no error set?

[Daniel Chiaramello]

Hello.

I have a problem at one of our customers, who runs MySQL 5.5.8.

I have something like the following code:

MYSQL_RES *res;
MYSQL_ROW row;
MYSQL* mysql = XXX();
uint32 ret;
char* sqlQuery = "SELECT MAX(id) FROM mytable";

ret = mysql_real_query(mysql, sqlQuery, (unsigned int)strlen(sqlQuery));
if (ret != 0)
{
  printf("### Error mysql_real_query: '%s'", mysql_error(mysql));
  return;
}

res = mysql_use_result(mysql);
if (res == NULL)
{
  printf("### Error mysql_use_result: '%s'", mysql_error(mysql));
  return;
}
...

But I have the following result:
### Error mysql_use_result: ''

I don't understand why mysql_use_result would return NULL and mysql_error is not set. By the way, there is also no reason why mysql_use_result returns NULL (the table is present and the request is returning value when run manually), but that's another story...

Did you already have this problem, of having a function returning an error but the error is not set??? Where could I look to find more investigation elements?

Thanks for your attention,
Daniel