Help with understanding Hopcroft's algorithm
Hopcroft's algorithm works well on complete DFA. (ie. for $M=\{Q,\Sigma,\delta,q_0,F\}$, the transition function $\delta:Q\times\Sigma\to Q$ is a total function).

For incomplete DFA with partial state transition function $\delta$. You can always convert it to complete DFA $M'=\{Q',\Sigma,\delta',q_0,F\}$ where $$Q'=Q\cup\{q_{err}\}$$ and $\delta'$ extends $\delta$ by define $\delta_a(q)=q_{err}$ for all undefined $\delta_a(q), \delta_a\in\delta,q\in Q\cup\{q_{err}\}$.