R中使用nls做指数下降拟合，当y中含有NA时，不返回对应的预测值，如何解决
x <- c(0.25,  1.00,  4.00, 11.00, 18.00, 25.00, 32.00, 56.00)
y <- c(27.017,     NA, 26.497, 25.706, 25.430, 25.481, 25.299, 25.322)
data.xy <- as.list(cbind(x,y))
xy.nls <- nls(y ~ a * exp(-1 * b * x) + c, data = data.xy, start = list(a = 9, b = 0.004, c = 18))
summary(xy.nls)
xy.nls.pred <- predict(xy.nls, newdata = data.xy)

xy.nls.pred只返回了7个值，NA对应的值没有，如何解决？