C++ Help，a memory access error when using memset in a three-dimensional array instantiate with a custom class template

Question

I'm new to C++and I'm learning it now,but I meet a problem when I tried to using memset in an 3D array class I defined. Here's the defination of the class.

template <class T>
class CArray3D
{
    T*** array;
    int l1;
    int l2;
    int l3;
public:
    CArray3D(int l1,int l2,int l3):l1(l1),l2(l2),l3(l3){
        array = new T**[l1];
        for(int i = 0;i < l1;++i)
        {
            array[i] = new T*[l2];
            for(int j = 0;j < l2;++j)
            {
                array[i][j] = new T[l3];
            }
        }
    };
    T** operator[](int i){
        return array[i];
    }
};

and the memset use.

CArray3D<int> a(3,4,5);
// initialize array a with for-loop
memset(a[1],-1 ,20*sizeof(int));
//try to print it with for-loop but met memory access problem
memset(a[1],-1 ,20*sizeof(int));    
memset(a[1][1],0 ,5*sizeof(int));

Exception: EXC_BAD_ACCESS (code=1, address=0xffffffffffffffff)

I'm debuging it in mac arm with clang++ lldb.

and I didn't meet the problem when using built-in 3D array a[][][] in same test code.