FORMATting a decimal into a string

We've got a column that's going to be one of two things:
- If a Work Center No. is available, show it
- Otherwise, show the Reserved Quantity

So what I'm doing is creating a single global variable called internalRsvQty, making it a string and setting it to the value of one or the other, like so:

IF "Work Center No." = '' THEN
  internalRsvQty := FORMAT("Reserved Quantity")
ELSE
  internalRsvQty := "Work Center No.";

When I print my form all I get are 0's (zeros) everywhere except where there should be a Work Center No. -- that's actually displaying correctly. But the Reserved Quantity just seems to be getting ignored. What am I doing wrong?