R：缺少值，其中幸存对象需要TRUE / FALSE

Question

library(survival)

> dput(dummy)
structure(list(time = c(-12, -12, -12, -12, 6, 6, 24, 0, 0, 12, 
48), event = c(1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 1)), .Names = c("time", 
"event"), row.names = 82:92, class = "data.frame")

> dummy
   time event
82  -12     1
83  -12     1
84  -12     1
85  -12     1
86    6     1
87    6     1
88   24     1
89    0     1
90    0     1
91   12     0
92   48     1

> mysurv = survfit(Surv(time, event) ~ 1, data = dummy)

> summary(mysurv)
Call: survfit(formula = Surv(time, event) ~ 1, data = dummy)

Error in if (any(mat[, ncol(mat) - 1] > mat[, ncol(mat)])) { : 
  missing value where TRUE/FALSE needed

当我在summary对象上调用survfit时出现此错误。可能是什么导致了这个？