Survival analysis: “X observations deleted due to missingness”

Dear r/rstats community. I have to perform a cox regression, but wanted to conduct some basic survival analysis before I do. I’m trying to run a basic survival curve via survfit, but am told that X observations were deleted due to missingness.

Code applied:

survfit(Surv(survivaltime, event) ~ exposure, data = data)

I have checked whether there are missings in either of the variables included, but there are none, and therefore I suspect that R is omitting observations because there are NA’s in general throughout the dataset. Am I correct and how do I approach this?

Hoping there is someone out there who can help! :-)