C++ - copying vector in foreach gives "No matching function to call for std::vector

So I have the following function:

std::vector<std::vector<int>> solve(int t){
    std::vector<std::vector<int>> result;
    result.push_back(std::vector<int>(2*t,0));
    //CODE TO fill up result[0]
    return result;
}

And when I write the following code to get results:

std::vector<std::vector<int>> results(4);
for(int t = 0; t < 4; ++t){
    std::vector<std::vector<int>> cols = solve(t);
    if(cols.size() > 0){
        for(std::vector<int> col: cols){
            results[t].push_back(col);            
        }
    }
}

I get the following error:

src/pricing.cpp:33:29: error: no matching function for call to ‘std::vector::push_back(std::vector&)’ results[t].push_back(col);

From what I understand the foreach is creating col as a reference. What I don't understand is why push_back being able to insert col. Why is this happening and what's the best way to insert col into results[t]?