What does count Dracula warn Johnathan not to do when he is alone at night?

Count Dracula warns Johnathan Harker not to leave the castle at night when he is alone. This is stated explicitly in the novel *Dracula* by Bram Stoker:

"He [Dracula] said he would not be able to see me for some time, and that I must not leave the castle on any pretext."

This is a major point of conflict in the novel as Johnathan is, of course, curious about the castle and its surroundings and eventually does leave at night, leading him to discover more about Dracula's true nature.