arithmetic problem?

[fachat]

When I write

       ldx $d900,y           
       lda VIDPAGE<<8+$900,y 
       sta $d900,y           
       txa                   
       sta VIDPAGE<<8+$900,y 

I get

.C:9344  99 00 E0    STA $E000,Y
.C:9347  BE 00 D9    LDX $D900,Y
.C:934a  B9 00 E0    LDA $E000,Y
.C:934d  99 00 D9    STA $D900,Y

So the shift left of VIDPAGE is done, but the addition of the offset is missing