Consider addi x5, x0, 0xf0f. The I-type instruction takes 12-bit immediates, which are sign-extended. Probably because of the sign extension, venus disallows unsigned immediates which occupy the 12th bit. It should be allowed to use these kinds of immediates with I-type instructions (and for that matter, full 20-bit unsigned immediates with U-type etc.), especially when we only care about the lowest 12 bits.