Why do I get incorrect count results for my sqlite query?

I have a sqlite query as follows:

select msg_id, frag_nr from fragments;

which, in python returns the following results:

[(u'i2uhru3hru3', 20)]

Now I am trying to count the number of rows that have the same 'msg_id' and 'frag_nr' using the following query:

select count(*) from fragments where 'msg_id'=='i2uhru3hru3' and 'frag_nr'=='20';

which returns the result

[(0,)]

i.e. zero matching rows. Do I use some wrong syntax, or why do I get this results, what seems to me incorrect. I expect to get a count of '1' instead of '0'...