heap corruption detected | C++

I get this "heap corruption detected" message after running this code :

uli& uli::operator =(char* n)
{
    char* buffer = new char[strlen(n)];

    char* p;
    int op;
    int coef;

    strcpy(buffer, n);

    while(*buffer)
    {
        op = strlen(buffer) - 5;
        p = (op >= 0) ? op+buffer : buffer;
        coef = atoi(p);

        if(coef > 65535)
            coef = atoi(++p);

        push(head, coef);
        *p = '\0';
    }

    delete buffer;       //  <- heap corruption detected

    return *this;
}

This is how I call the method:

uli x;
x = "9876123";

What does "heap corruption detected" mean ?