It's hard to say without seeing the full code, but:
k have a sensible value? Moreover, if
TUnaire can contain k+1 chars as you wrote it,
TUnaire[k+1] is one past the end of
TUnaire (and it's an error).
Check the return value of malloc (
Total != NULL).
TUnaire are not valid pointers, or
k is negative or too large, or there is bug elsewhere in your code.