Can't query system tables during transaction

Hi, ALL,
I'm trying to execute following:

SELECT 1 FROM pg_class c, pg_namespace n WHERE n.oid = c.relnamespace
AND c.relname = 'foo' AND n.nspname = public;

inside the transaction.

I'm getting the following error:

ERROR: current transaction is aborted, commands ignored until end of
transaction block

Does this mean I can't query system tables during the transaction?
What is the problem here if it's not and how do I find out the reason?
And if it is - how to work around it?

I can probably commit it and start a new transaction, but I fear I will
have the same issue there...

Thank you.

If it matters - I'm working with C++ and libpq.