View Compilation Error

0_0_39512722_1340.cpp: In function 'std::string karatsubaMultiply(const std::string&, const std::string&)':
0_0_39512722_1340.cpp:61:172: error: invalid initialization of reference of type 'const std::vector<int>&' from expression of type 'std::string' {aka 'std::__cxx11::basic_string<char>'}
   61 |     vector<int> c1 = addVectors(stringToVector(karatsubaMultiply(vectorToString(addVectors(a1, a0)), vectorToString(addVectors(b1, b0)))), vectorToString(karatsubaMultiply(vectorToString(a1), vectorToString(b1))));
      |                                                                                                                                                           ~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
0_0_39512722_1340.cpp:39:42: note: in passing argument 1 of 'std::string vectorToString(const std::vector<int>&)'
   39 | string vectorToString(const vector<int> &v) {
      |                       ~~~~~~~~~~~~~~~~~~~^