GBA: Issue drawing sprite object over background

Question

I'm trying to write a very basic GBA game.

I have been reading the tonc examples and am using the tonc lib. I have something that builds and runs, but I am failing to get a sprite to draw over a background. At the moment, the background renders ok, but the object over the top just appears as a glitchy pink square in the top left of the screen.

    int main ()
    {
        initMain();
    
        while(1)
        {
            vid_vsync();
            key_poll();
    
            updateMain();
        }
    
        return 0;
    }

    void initMain()
    {
        // background
        memcpy(pal_bg_mem, backgroundPal, backgroundPalLen);
        memcpy(&tile_mem[0][0], backgroundTiles, backgroundTilesLen);
        memcpy(&se_mem[30][0], backgroundMap, backgroundMapLen);
    
        // character
        memcpy(pal_obj_mem, characterPal, characterPalLen);
        memcpy(&tile_mem[4][0], characterTiles, characterTilesLen);
    
        oam_init(obj_buffer, 128);
    
        REG_BG0CNT = BG_CBB(0) | BG_SBB(30) | BG_8BPP | BG_REG_32x32;
        REG_DISPCNT = DCNT_MODE0 | DCNT_BG0 | DCNT_OBJ | DCNT_OBJ_1D;
    }
    
    void updateMain()
    {
        // character
        int x = 96, y = 96;
        u32 tid = 0, pb = 0;
    
        OBJ_ATTR *character = &obj_buffer[0];
        obj_set_attr(character, 
            ATTR0_SQUARE,               // Square, regular sprite
            ATTR1_SIZE_64,              // 64x64p, 
            ATTR2_PALBANK(pb) | tid);   // palbank 0, tile 0
    
        character->attr2 = ATTR2_BUILD(tid, pb, 2);
    
        obj_set_pos(character, x, y);
        oam_copy(oam_mem, obj_buffer, 1);
    
        REG_DISPCNT = DCNT_MODE0 | DCNT_BG0 | DCNT_BG1 | DCNT_OBJ | DCNT_OBJ_1D ;
    }

I am no doubt making a basic mistake and just need a little bit of help to understand what I've done wrong here.