PVS-Studio warning V635 for WinMerge

int
iconvert_new (LPCTSTR source, LPTSTR *destination,
              int source_coding, int destination_coding,
              bool alphabet_only)
{
  LPTSTR dest = (LPTSTR) malloc (_tcslen (source) + 1 + 10);
  int result = -3;
  if (dest)
  {
    _tcscpy (dest, source);
    ....
}

This suspicious code was found in WinMerge project by PVS-Studio static code analyzer.
Warning message is:
V635 Consider inspecting the expression. The length should probably be multiplied by the sizeof(wchar_t). cs2cs.cpp 236

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