R: dplyr pipe conditional lead/lag using ifelse with unexpected behavior

I'm trying to use a conditional lead/lag function in a dplyr pipe using ifelse but getting an error. However, using the same approach outside the pipe it seems to work. What am I missing?
Version info:
R 3.1.1
dplyr_0.4.1

require(dplyr)

Data:

test <- data.frame(a = c("b","b","b","b","b","b",
                         "m","m","m","m","m","m",
                         "s","s","s","s","s","s"), 
                   b = replicate(1,n=18), 
                   stringsAsFactors=F)

Dplyr pipe:

test %>%
  mutate(delta = ifelse(a=="s",b+lag(b,n=2*6),
                        ifelse(a=="m",b+lag(b,n=1*6),0)))

--> Error: could not convert second argument to an integer. type=LANGSXP, length = 3

Without the pipe it works:

test$delta <- ifelse(test$a=="s",test$b+lag(test$b,n=2*6),
                     ifelse(test$a=="m",test$b+lag(test$b,n=1*6),
                            0))

I found some indication that there was an issue with dplyr lead/lag in combination with grouped data frames. But I am not grouping here.