strtok function returns NULL value

treadbuff value is 0x06818A30 "" However, strtok function returns NULL value so error happens in the last row.

I don't know what is wrong here.

#define MAX_COMM_LEN 128   
char *tbuff; 
char *treadbuff;  
double tcurrent;     

treadbuff = (char*)malloc(MAX_COMM_LEN*sizeof(char)); 

memset(treadbuff, 0x0, MAX_COMM_LEN);  

tbuff = (char*)malloc(MAX_COMM_LEN*sizeof(char));               
memset(tbuff, 0x0, MAX_COMM_LEN);

tbuff = strtok (treadbuff, "A,");
tcurrent = atof(tbuff);