This is a quote from "Dune" by F. Herbert:
"They'll not watch you in this thing," the Duke said. "You'll keep the secret. You must." He walked to the windows, spoke without turning. "This way, if anything should happen to me, you can tell her the truth—that I never doubted her, not for the smallest instant. I should want her to know this."
Technically I understand, of course, what it means, but is it grammatically correct?