You're most likely using an invalid form:
Code
LDA $0100,y ; exists, okay to use LDA $00,y ; does not exist LDA $7F0000,y ; does not exist
I recommend checking this page for a list of valid opcodes. For the most part, Y indexing only exists in the address form.
(the only exception to this, other than indirect indexing, is LDX dp,y for some reason)
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