ASSIGN not working in initialization file

I'm trying to get COM1 to map to S1IN, but am having a few problems. I have the following in an initialization file

MODE COM1 4800, 0, 8, 1
ASSIGN WIN2 <NUL >S1OUT
ASSIGN COM1 <S1IN >NUL

However, when I enter debug mode and then type assign I get the following output

MODE COM1 4800, 0, 8, 1
ASSIGN WIN2 <NUL >S1OUT
ASSIGN COM1 <S1IN >NUL
ASSIGN
  WIN: <S0IN >S0OUT
 WIN2: <NUL >S1OUT
 WIN3: <NUL >NUL
 COM1: <NUL >NUL
 COM2: <NUL >NUL
 COM3: <NUL >NUL
 COM4: <NUL >NUL
 COM5: <NUL >NUL
 COM6: <NUL >NUL
 COM7: <NUL >NUL
 COM8: <NUL >NUL

If I now try entering the commands through the command line interface, it seems to work, giving the following output

MODE COM1 4800, 0, 8, 1
ASSIGN WIN2 <NUL >S1OUT
ASSIGN COM1 <S1IN >NUL
ASSIGN
  WIN: <S0IN >S0OUT
 WIN2: <NUL >S1OUT
 WIN3: <NUL >NUL
 COM1: <S1IN >NUL
 COM2: <NUL >NUL
 COM3: <NUL >NUL
 COM4: <NUL >NUL
 COM5: <NUL >NUL
 COM6: <NUL >NUL
 COM7: <NUL >NUL
 COM8: <NUL >NUL

What am I doing wrong? Why is the initialization file not working on it's own?

Thanks