Zoho Creator Tutorial #3 error message

Zoho Creator Tutorial #3 error message

I am working through Creator tutorial 3 and am attempting to write the deluge code to decrease inventory on success.  Code is below:

for each itemID in input.Product
{productRow=Product[ID == itemID];
productRow.Available_Stock = (productRow.Available_Stock -1);
}

I am getting an error message stating "Only List Expressions are allowed to iterate, but (input Product) is of type STRING

I am sure the code is identical to the tutorial and can't find why this would be wrong.  Any help would be appreciated.

John