Sсhur's lemma states:
Lemma. Let $R$ be a finite-dimensional algebra over an algebraically closed field $K$ and let $M$ be a simple $R$-module. Then $\textrm{End}_R(M)\cong K.$
Proof. Since $M$ is finitely generated, it's clearly finite-dimensional over $K.$ Consider $\Psi\in\textrm{End}_R(M).$ It is a $K$-linear operator. Let $\alpha$ be an eigenvalue of $\Psi.$ Then $\Psi-\alpha\cdot\textrm{Id}$ has got no inverse. Hence, $\Psi=\alpha\cdot\textrm{Id}.$
I want to prove the following:
Proposition. Let $$ be a $\mathbb C$- algebra and let $$ be a simple $$-module with a countable basis (over $\mathbb C$). Then $\textrm{End}_R(M)\cong \mathbb C.$
Here one cannot refer to the finiteness of the dimension. How to get around this difficulty?