Error : "Property 'value' does not exist on type 'string'."

Hello !

I have an object’s collection and I try to get a specific value. Something like this :
this.field = this.field + this.selectedOptions[i].value;

When I do ionic serve, I have this error in my console :
"Property ‘value’ does not exist on type ‘string’."
I don’t understand because the value exists.

I have this error but my app works … sometimes. Without knowing, the code can stop to run and the error become fatal.

Someone have an idea ?