Push down automata what to do when there is no suitable transition
Starting with empty stack is not allowed.

A PDA is usually defined as (Q, &Sigma;, &Gamma;, &delta;, q0, Z0, F), where the symbols have their usual meanings. One of these is Z0, the stack start symbol. It is on the stack when the PDA starts.

So by definition you have Z0 on the stack at start.
You need to find Z0 in your case and start with that.