How does toString() work for native types?

[U-100]

I'm using Jint in Unity, there is the type Unity.Mathematics.int2 which I expose to jint.
Doing so works:
let x = "" + (new int2(1, 1)).toString();
But doing so throws an exception:
let x = "" + new int2(1, 1);
Why is that?

[lahma]

This might just be something that is not implemented, depending on the error.

[U-100]

Here's what I get:
TargetInvocationException: Exception has been thrown by the target of an invocation.
Jint.Runtime.ExceptionHelper.ThrowMeaningfulException (Jint.Engine engine, System.Reflection.TargetInvocationException exception) (at :0)
Jint.Runtime.Interpreter.Expressions.JintBinaryExpression.TryOperatorOverloading (Jint.Runtime.Interpreter.EvaluationContext context, Jint.Native.JsValue leftValue, Jint.Native.JsValue rightValue, System.String clrName, System.Object& result) (at :0)
Jint.Runtime.Interpreter.Expressions.JintBinaryExpression+PlusBinaryExpression.EvaluateInternal (Jint.Runtime.Interpreter.EvaluationContext context) (at :0)
Jint.Runtime.Interpreter.Expressions.JintExpression.Evaluate (Jint.Runtime.Interpreter.EvaluationContext context) (at :0)
Jint.Runtime.Interpreter.Expressions.JintExpression.GetValue (Jint.Runtime.Interpreter.EvaluationContext context) (at :0)
Jint.Runtime.Interpreter.Statements.JintVariableDeclaration.ExecuteInternal (Jint.Runtime.Interpreter.EvaluationContext context) (at :0)
Jint.Runtime.Interpreter.Statements.JintStatement.Execute (Jint.Runtime.Interpreter.EvaluationContext context) (at :0)
Jint.Runtime.Interpreter.JintStatementList.Execute (Jint.Runtime.Interpreter.EvaluationContext context) (at :0)
Jint.Engine.ScriptEvaluation (Jint.Runtime.ScriptRecord scriptRecord, Acornima.ParserOptions parserOptions) (at :0)
Jint.Engine+<>c__DisplayClass99_0.b__0 () (at :0)
Jint.Engine.ExecuteWithConstraints[T] (System.Boolean strict, System.Func1[TResult] callback) (at <b6373b963bab4449aded3d8de77ecdda>:0) Jint.Engine.Execute (Jint.Prepared1[Acornima.Ast.Script]& preparedScript) (at :0)
Jint.Engine.Evaluate (Jint.Prepared`1[Acornima.Ast.Script]& preparedScript) (at :0)
Jint.Engine.Evaluate (System.String code, System.String source) (at :0)

[U-100]

That happens if I try to
engine.Evaluate("let x = \"\" + new int2(1, 1);");