what this means ?

Func compilation error: contracts/3.fc:35:22: error: cannot apply function ~push_back : (tuple, cell) -> (tuple, ??9) to arguments of type (tuple, int): cannot unify type int with cell

here is the code :
tuple find_and_replace(int flag, int value, cell linked_list) method_id {
slice s = linked_list.begin_parse();

tuple stack = null();
stack~push_back(linked_list);

tuple res = empty_tuple();
while (~ stack.is_null()) {
slice cs = stack~pop_back().begin_parse();
int ci = cs~load_uint(8);

res~push_back(ci);

repeat (current_slice.slice_refs()) {
stack~push_back(current_slice~load_ref());
}
}