Ah - language ...
If *you* write "if $R \| C_i$ has valid padding", you imply for me that the *message* has padding (R || C_i is a complete message, consisting of two blocks). Then, my language feeling seems to tell me, that a text that continues with "such a" *must* be followed by something of the same kind - i.e. a "message of multiple blocks", not a "block".
If you want to say "such a random plaintext *block*", you need find something in the previous sentence that *is* a block - so, you must write "if C_i has valid padding" (which is not exactly what you want to say, because R takes part in that validity of the padding".
Or maybe you want to refer to the "target block" of the sentence *before* the last one. Then you must "redirect" the reference - "a random block as our C_i" or the like.
... just what I feel.
(Regarding correctness, of course both a message and its last block can be said to have been valid or invalid padding - that's not the issue).