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I was playing my sliver deck against the computer's Red Cats deck. I had a number of slivers on the board, including Shifting Sliver. The computer had a few creatures out, including at least one Guma. There was nothing else on the board that would have made the slivers blockable. I attacked with all of my slivers that were capable of attacking at that point. The computer blocked a Blade Sliver with a Guma, which should be an invalid block, as the Guma was not a sliver. Even the protection from blue that the Guma had should not have allowed it to block.