Realloc: invalid next size detected by glibc

My code is:

int args_size = 5;
char ** args;

args = (char **) malloc (sizeof (char *) * args_size);

// ...

args = (char **) realloc (args, sizeof (char *) * (args_size + = 5));

I want to increase the size by 5.

But I get this error:

*** glibc detected *** ./a.out: realloc (): invalid next size: 0x0000000000a971c0 ***

I know the catching realloc temporary variable is good, but just for simplicity ...