SQL result doesn't work when reassigned

[bermejo]

The code:\[code\] $msr = db_query("SELECT * FROM users WHERE username='$username'"); if (db_num_rows($msr) == 0) return null;\[/code\]When\[code\]function db_query($query) { return mysql_query($query) or die(mysql_error() . " when querying: $query"); }function db_num_rows($queres) { return mysql_num_rows($queres) or die(mysql_error()); }\[/code\]Shows error: \[code\]Warning: mysql_num_rows(): supplied argument is not a valid MySQL result resource\[/code\]When I replace "db_" with "mysql_" everything works perfectly.Is there a way to fix this?