catastrophic breakdown of gen codebox

codebox keeps on giving!
I'm trying to generalise my data arrays to more than one channel (to store several instances of my data), given as a parameter (default 1).

Is this way to define the number of channels actually possible?:

Param size;
Data mydata(21, size);

If so, there's another problem in the gen compiler, the below code produces an assertion:

gen_domain: dsp.gen: [string "gen2.dsp.Model"]:1455: assertion failed!
gen: failed to compile patcher

get_index_from_id (id, id_to_index, inst)
{
    return peek(id_to_index, id, inst);
}

get_unused_index (id, index_to_id, inst)
{
    // find first free index
    for (index = 0; index < dim(index_to_id); index += 1)
    {
         if (peek(index_to_id, index, inst) < 0)
            break; // free slot found (-1)
    }
    return index;
}

add_id (id, count, index_to_id, id_to_index, inst)
{
    index = get_unused_index(id, index_to_id, inst);
    // poke(v, i) writes value v at position i (!)
    poke(index_to_id, id,    index, inst);
    poke(id_to_index, index, id,    inst);
    addit = (index < dim(index_to_id));
    poke(count, peek(count, inst) + addit, inst);
    return index;
}

remove_id (id, index, count, index_to_id, id_to_index, inst)
{   // clear array slots: set to -1
    poke(index_to_id, -1, index, inst);
    poke(id_to_index, -1, id,    inst);
    poke(count, peek(count, inst) - 1, inst);
}

Param instances; // number of independent alive lists

Data index_to_id (32, instances);
Data id_to_index (32, instances);
Data count (instances);

History clear(1);

if (clear) 
{
    // do this at init, or when clear flag is set from outside
    n = dim(index_to_id);
    m = channels(index_to_id);

    for (k = 0; k < m; k += 1)
    {
        for (i = 0; i < n; i += 1) 
        {   // set all arrays to -1 meaning empty slot
            poke(index_to_id, -1, i, k);
            poke(id_to_index, -1, i, k);
        }
        poke(count, 0, k);
    }
    clear = 0;
}    

// input: 
// in1: integer ID
// in2: 'activation' value (velocity or touch phase)
id  = in1;
act = in2;
// in3 is flag how to interpret activataion:
// - 0: as velocity, 
// - 1: as ROLI touch phase (0 - begin, 1 - middle, 2 - end)
on  = in3 == 0  ?  act > 0  :  act != 2;
inst = in4; // current instance number = channel

// check if id is already known
index = get_index_from_id(id, id_to_index, inst);

if (index < 0)
{ // no: find first unused index and add id
    index = add_id(id, count, index_to_id, id_to_index, inst);
}    

// check if id switches off
if (!on)
{   // clear slots
    remove_id(id, index, count, index_to_id, id_to_index, inst);
}

out1 = index;
out2 = peek(count, inst);