On debian platform, checking the class gives a vector of length 2

Currently, on debian platform, checking the class of an R object, for e.g., class(Diag), somehow gives a vector of length 2. And when it is fed to if condition statement, it gives _R_CHECK_LENGTH_1_LOGIC2_ ( 'length(x) = 2 > 1' in coercion to 'logical(1)' ) error.

