python - django-haystack - filter not working for value 'a', but working for other letters and words -

i have index this;

class indexclass(searchindex):     text = charfield(document=true, use_template=true)     f1 = charfield(model_attr='f1') 

lets say, f1 has values 'a', 'b', 'c', 'sdfsdf', 'sd' (different lengths of letters, words). able filter this:

searchqueryset.filter(f1='b') (not 'b', other except 'a' working) 

but, when try letter 'a', not working.

where think problem ?

by way, tried modify values manytimes, , when modify value 'a', not working.