realloc array of struct inside another struct

I'm working on a project where the goal is to create automatons.

Automatons are defined by a struct :

typedef struct {
    int num_states;
    int initial_state;
    State * states;
} Automaton;

And State is another struct defining arcs between states:

typedef struct {
    int num_arcs;
    bool is_final;
    Arc * arcs;
} State;

typedef struct {
    int symbol;
    int destination;
} Arc;

I create an automaton with malloc as such :

Automaton* create_automaton(void) {
    Automaton * a = (Automaton *)malloc(sizeof(Automaton));
    assert(a != NULL);
    a->initial_state = UNDEFINED;
    a->num_states = 0;
    a->states = NULL;
    return a;
}

So then I want to take 2 Automatons with states and arcs created with these functions :

int add_state(Automaton* a) {
    State* state = (State *)realloc(a->states, (a->num_states + 1) * sizeof(State));
    if(state == NULL)
        exit(EXIT_FAILURE);
    a->states = state;
    a->states[a->num_states].num_arcs = 0;
    a->states[a->num_states].is_final = FALSE;
    a->states[a->num_states].arcs = NULL;

    return a->num_states++;
}

void add_arc(Automaton* a, int from, int to, int symbol) {
    if(from >= a->num_states || to >= a->num_states)
        exit(EXIT_FAILURE);
    Arc * arc = (Arc *)realloc(a->states[from].arcs, (a->states[from].num_arcs + 1) * sizeof(Arc));
    if(arc == NULL)
        exit(EXIT_FAILURE);
    a->states[from].arcs = arc;
    a->states[from].arcs[a->states[from].num_arcs].destination = to;
    a->states[from].arcs[a->states[from].num_arcs].symbol = symbol;
    a->states[from].num_arcs++;
}

I want to combine these 2 Automatons in one so I wrote this function:

Automaton* append_automaton(Automaton * a1, Automaton * a2)
{
    Automaton * a = copy_automaton(a1);
    int i = 0;
    for(i = 0; i < a2->num_states; i++)
    {
        add_state(a);
        a->states[a1->num_states + i] = a2->states[i];
        for(j = 0;j<a->states->num_arcs;j++)
        {
            a->states[i].arcs[j].destination =+ a2->num_states;
        }
    }
    a->initial_state = a1->initial_state;

    return a;
}

However I can create the Automaton, add states and arcs to it without any problem, when i try to merge them together with append_automaton I get a segmentation fault when in add_state() I realloc State to fit one more state in the new automaton.

So my question is the following : Why is realloc giving me a segmentation fault when in this function (append_automaton) althought it works perfectly outside of it?

PS: copy_Automaton() does indeed overwrite the create_Automaton() so I removed the line: Automaton * a = create_automaton() in append_automaton() And here is copy_automaton():

Automaton* copy_automaton(Automaton* a) {
    int i = 0;
    Automaton * cp_a = malloc(sizeof(Automaton));
    cp_a->states = malloc(sizeof(a->states));
    for(i = 0; i < a->num_states; i++)
    {
        cp_a->states[i].arcs = malloc(sizeof(a->states[i].arcs));
        cp_a->states[i] = a->states[i];
    }
    cp_a->num_states = a->num_states;
    cp_a->initial_state = a->num_states;
    //memcpy(a, cp_a, sizeof(Automaton));
    return cp_a;
}