Hello, I was wondering if someone could tell me why my function is not working. Here is my function that I have: 

1. 
   
2. String Bar_Code(String X)
   
3. {
   
4. //remove serial
   
5. if(X.contains("--"))
   
6. {
   
7. sku4 = X.getPrefix("--");
   
8. }
   
9. else
   
10. {
    
11. sku4 = X;
    
12. }
    
13. //Find Book 
    
14. if(sku4.contains("97816804") == true)
    
15. {
    
16. Book3 = Books[Barcode == sku4];
    
17. }
    
18. else if(sku4.contains("9781942") == true)
    
19. {
    
20. Book3 = Books[Barcode == sku4];
    
21. }
    
22. else if(sku4.contains("9780934") == true)
    
23. {
    
24. Book3 = Books[Barcode == sku4];
    
25. }
    
26. else
    
27. {
    
28. Book3 = Books[Book == sku4];
    
29. }
    
30.  return Book3;
    
31. }
    

 
But when I try to call it with this 

1.  Book3 = thisapp.Bar_Code(input.SKU);

I am getting an error that says invalid collection variable.