I am not getting how condition in if statement causing this error?

Line 1034: Char 9: runtime error: reference binding to null pointer of type 'int' (stl_vector.h)
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /usr/bin/../lib/gcc/x86_64-linux-gnu/9/../../../../include/c++/9/bits/stl_vector.h:1043:9

class Solution {
public:
    void merge(vector<int>& nums1, int m, vector<int>& nums2, int n) {
        int i=0;
        for(int j=0;j<nums1.size();j++){
            if(nums1[j]<nums2[i]){
                for(int k=m-i;k>j;k--){
                    swap(nums1[k],nums1[k-1]);
                }
                i++;
            }
        }
    }
};