How do I set a random number of random lowercase characters to a Struct member using memset in C

I am forced to use memset and drand48() to set a random number (2 - 7) of random characters that are lower case letters ('a' to 'z'). My code returns non ASCII characters and I am not sure why.

struct Record {
    int seqnum;
    float threat;
    unsigned int addrs[2];
    unsigned short int ports[2];
    char dns_name[NUMLTRS];
};

My code is in a for loop:

memset(rec_ptr[i].dns_name, (char)((122 * drand48()) + 97), 
((sizeof(char) * 7) * drand48()) + (sizeof(char) * 2));