CID 1508975: Argument cannot be negative

[maskit]

101int
102get_redis_auth_key(char *retKeyBuff, int buffSize)
103{
104  int retval = 0;
105  // Get the Key
   	1. Condition ssl_param.redis_auth_key_file.length(), taking true branch.
106  if (ssl_param.redis_auth_key_file.length()) {
   	2. negative_return_fn: Function open(ssl_param.redis_auth_key_file.c_str(), 0) returns a negative number.
   	3. assign: Assigning: fd = open(ssl_param.redis_auth_key_file.c_str(), 0).
107    int fd = open(ssl_param.redis_auth_key_file.c_str(), O_RDONLY);
108    struct stat info;
   	
CID 1508975 (#1 of 1): Argument cannot be negative (NEGATIVE_RETURNS)
4. negative_returns: fd is passed to a parameter that cannot be negative. [Note: The source code implementation of the function has been overridden by a builtin model.]
109    if (0 == fstat(fd, &info)) {

[duke8253]

I'm not sure what it means by "fd is passed to a parameter that cannot be negative" for line 109. fstat() is defined as int fstat(int fildes, struct stat *buf); https://man7.org/linux/man-pages/man3/fstat.3p.html .

Or is it actually saying fd should be checked on line 107?