Question: I am having a challenge folding a list of integer elements to sum them up.

```
 List<int> ints = [1,2,3,4,5];
 var result = ints.fold(0, (pV,element)=>pV + element);
 print(result);

```

Somehow I can't get this simple code to compile. I keep getting the error:

```
Error compiling to JavaScript:
Warning: Interpreting this as package URI, 'package:dartpad_sample/main.dart'.
lib/main.dart:19:46:
Error: The operator '+' isn't defined for the class 'Object?'.
 - 'Object' is from 'dart:core'.
var result = ints.fold(0, (pV,element)=>pV + element);
                                         ^
Error: Compilation failed.

```

Any help

Answer: 
I don't know why, but if you are explicit with the types, it works:

```
var result = ints.fold(0, (int pV, element) => pV + element);

```
As **pV** is of type object and **element** is of type int, you cannot add them.

You can do something like this if you required:

```
List<int> ints = [1,2,3,4,5];
 var result = ints.fold(0, (pV,element)=>int.parse(pV.toString()) + element);
 print(result);

```