View Compilation Error

0_0_27534312_21323.cpp: In function 'int main()':
0_0_27534312_21323.cpp:63:9: error: no match for 'operator=' (operand types are 'BigInt' and 'BigInt')
   fn[i] = add(fn[i - 2], fn[i - 1]);
         ^
0_0_27534312_21323.cpp:63:9: note: candidate is:
0_0_27534312_21323.cpp:20:10: note: BigInt& BigInt::operator=(BigInt&)
  BigInt& operator=(BigInt& a){
          ^
0_0_27534312_21323.cpp:20:10: note:   no known conversion for argument 1 from 'BigInt' to 'BigInt&'