  A lower bound on the maximum likelihood (ML) decoding error exponent of
linear block code ensembles, on the erasure channel, is developed. The lower
bound turns to be positive, over an ensemble specific interval of erasure
probabilities, when the ensemble weight spectral shape function tends to a
negative value as the fractional codeword weight tends to zero. For these
ensembles we can therefore lower bound the block-wise ML decoding threshold.
Two examples are presented, namely, linear random parity-check codes and
fixed-rate Raptor codes with linear random precoders. While for the former a
full analytical solution is possible, for the latter we can lower bound the ML
decoding threshold on the erasure channel by simply solving a 2 x 2 system of
nonlinear equations.
