i cannot get why am i getting this error continously:

Line 1034: Char 34: runtime error: applying non-zero offset 24 to null pointer (stl_vector.h) SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_vector.h:1043:34

class Solution { public: int numIdenticalPairs(vector& nums) {

    int pair_count=0;
    vector<vector<int>> v;
    for(int i=0;i<nums.size();i++)
    {
        v[nums[i]].push_back(i);
    }
    
    for(int i=1;i<v.size();i++)
    {
        int size=v[i].size();
        pair_count+=floor(size/2);
    }
    
    return pair_count;
}

};