R version 3.1.1 (2014-07-10) -- "Sock it to Me"
> bl <- c(140, 138, 150, 148, 135)
> fu <- c(138, 136, 148, 146, 133)
> t.test(fu, bl, alternative = "two.sided", paired = TRUE)
Error in t.test.default(fu, bl, alternative = "two.sided", paired = TRUE) :
data are essentially constant

> fu <- c(138, 136, 148, 146, 132)

> t.test(fu, bl, alternative = "two.sided", paired = TRUE)

    Paired t-test