SRILM BOW denominator warning

[David Gelbart]

Hello,

I am trying to build a trigram LM for the OGI Numbers corpus, in which 
utterances are spoken strings of numbers such as 'eighty nine eighty 
eight'.  Since there are no singletons, I am using Witten-Bell 
discounting instead of Good-Turing.  ngram-count displays "BOW 
denominator for context... is zero" warnings.  Does this mean the LM 
is broken?  If I try adding "-gt3min 1 -gt2min 1" to the ngram-count 
options, I still see these warnings.  Here is the ngram-count output:

$ ngram-count -wbdiscount -text /u/gelbart/tmp/train.trans -order 3 \
   -lm /u/gelbart/tmp/numbers-wb.lm
BOW denominator for context "seven" is zero; scaling probabilities to sum to 1
BOW denominator for context "six" is zero; scaling probabilities to sum to 1
BOW denominator for context "four" is zero; scaling probabilities to sum to 1
BOW denominator for context "two" is zero; scaling probabilities to sum to 1

In the generated language model, the log BOWs are zero for those four 
words:

-1.156247       four    0
-1.09725        seven   0
-1.203041       six     0
-1.029482       two     0

Thanks,
David