PVS-Studio warning V575 for Scilab

char **splitLineCSV(....)
{
  ....
  if (retstr[curr_str] == NULL)
  {
    *toks = 0;
    FREE(substitutedstring);
    substitutedstring = NULL;
    freeArrayOfString(retstr, strlen(substitutedstring));
    return NULL;
  }
  ....
}

This suspicious code was found in Scilab project by PVS-Studio static code analyzer.
Warning message is:
V575 The null pointer is passed into 'strlen' function. Inspect the first argument. splitline.c 107

PVS-Studio is a static analyzer for detecting bugs in the source code of applications written in C, C++, C++11, C++/CX. Site: http://www.viva64.com/en/pvs-studio/