Validate form based on subform fields not working

Validate form based on subform fields not working

Can anyone tell me why this doesn't work? It runs on validate of my Projects form, which has the subform Tasks_Subform.

An equivalent workflow run on validate of the Tasks form itself works perfectly, but the below code just seems to do nothing. The Projects form simply closes regardless of whether the conditions have been met and doesn't show the alert message.

Thanks in advance.

for each  row in input.Tasks_Subform
{
if(row.Closed = "Yes" && row.FY_complete_task = "")
{
alert "Specify the financial year the task was completed";
cancel submit;
}
}