empty character constant error

Can someone tell me what this error means and how can I fix it. I know it has to be something simple that I am overlooking. I have included the part of my program that contains the error:

Code:

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void NameFind(string carName[], string workOrder[], string mechanic[], int count, char let)
{
        int a;
        int b=0;
        cout << "\nCars with last name beginning with" << let << ":\n";

        for(a=0; a<count; ++a)
        {
                if(carName[a].find(toupper(let)) ==0)
                {
                        cout << "\nCar name:" << carName[a] << '' << "Work order:" << workOrder[a] << '' << "Mechanic:" <<mechanic[a];//error appears in this line????
                        ++b;
                }
        }
        if(b==0)
                cout << "There are no cars whose last name begins with " << let <<". \n";
}

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try replacing ' ' with " "