UDG definition bug (*solved*)

[wilco2009]

The ZXSpectrum hangs with the following code:

Code:

FOR n=0 TO 7: POKE USR "A"+n,255: NEXT n
FOR n=0 TO 7: POKE USR "B"+n,255: NEXT n
FOR n=0 TO 7: POKE USR "C"+n,255: NEXT n
FOR n=0 TO 7: POKE USR "D"+n,255: NEXT n
FOR n=0 TO 7: POKE USR "E"+n,255: NEXT n
FOR n=0 TO 7: POKE USR "F"+n,255: NEXT n

PRINT "\a \b \c \d \e \f"

Definition of UDG A & B works properly but program does not work from UDG C.