What is the reason for the memory leak?

int main () {
   char *str;

   /* /* Initial memory allocation */
   str = (char *) malloc(15);
   strcpy(str, "tutorialspoint");
   printf("String = %s,  Address = %u\n", str, str);
   str = NULL;

   free(str);

   return(0);
}

==4143== Memcheck, a memory error detector
==4143== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==4143== Using Valgrind-3.13.0 and LibVEX; rerun with -h for copyright info
==4143== Command: ./a.out
==4143== 
String = tutorialspoint,  Address = 86097984
==4143== 
==4143== HEAP SUMMARY:
==4143==     in use at exit: 15 bytes in 1 blocks
==4143==   total heap usage: 2 allocs, 1 frees, 1,039 bytes allocated
==4143== 
==4143== 15 bytes in 1 blocks are definitely lost in loss record 1 of 1
==4143==    at 0x4C2FB0F: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==4143==    by 0x1086EB: main (in /home/stack/a.out)
==4143== 
==4143== LEAK SUMMARY:
==4143==    definitely lost: 15 bytes in 1 blocks
==4143==    indirectly lost: 0 bytes in 0 blocks
==4143==      possibly lost: 0 bytes in 0 blocks
==4143==    still reachable: 0 bytes in 0 blocks
==4143==         suppressed: 0 bytes in 0 blocks
==4143== 
==4143== For counts of detected and suppressed errors, rerun with: -v
==4143== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)