Setting variable in if-case in Qt

I have a problem setting a variable in an if-case. Here's my code:

void ExcelResultFileWriter::setCombineP835(bool option){
    if(option == true){
        cout << "dario" << endl;
        this->combineSheets = true;
    }
    else{
        cout << "kein dario" << endl;
    }
}

I'm getting an address error when he tries to set the variable combineSheets. Does anyone have an idea why? When I'm only printing out the "dario" it works fine, but when it comes to setting the variable I'm getting the error. The variable is of course defined as

bool combineSheets

Thank you!