Due to the relative priorities of infix operators && and =, this is parsed as
(isinf(sdmove) && sdmove) = 999.99
instead of what you really meant:
isinf(sdmove) && (sdmove = 999.9)
You can use this last expression in order to explicitly set the correct order of operations.
(Opinions on this may vary from one developer to another, but I personally tend to dislike short circuits in cases where the RHS is an assignment, precisely for that reason. But I do like short circuits like isinf(sdmove) && break, because they don’t suffer from the same need for explicit parentheses)