ifelse in data.table agregarte result

I have a question which really need your help:

set.seed(1111)
s<-rep(seq(1,4),5)
a<-runif(20,0.2,0.6)
b<-runif(20,0.4,0.7)
b[6:8]<-NA
c<-runif(20,4,7)
d<-data.table(s,a,b,c)
setkey(d,s)

The data is as following:

    s         a         b        c
 1: 1 0.3862011 0.4493240 6.793058
 2: 1 0.4955267 0.4187441 4.708561
 3: 1 0.4185155 0.5916827 6.810053
 4: 1 0.5003833 0.5403744 5.948629
 5: 1 0.5667312 0.5634135 6.880848
 6: 2 0.3651699 0.5263655 5.721908
 7: 2 0.5905308        NA 6.863213
 8: 2 0.2560464 0.4649180 5.745656
 9: 2 0.4533625 0.5077432 5.958526
10: 2 0.4228027 0.4340407 5.115065
11: 3 0.5628013 0.6517352 6.252962
12: 3 0.5519840        NA 4.875669
13: 3 0.2006761 0.6418540 5.452210
14: 3 0.5472671 0.4503713 6.962282
15: 3 0.5601675 0.5195013 6.666593
16: 4 0.2548422 0.6962112 5.535579
17: 4 0.2467137        NA 6.680080
18: 4 0.4995830 0.6793684 6.334579
19: 4 0.2637452 0.4078512 6.076039
20: 4 0.5063548 0.4055017 5.287291

If I do a simple sum, using s as key, it will return a nice table summarize the result:

d[,sum(c),by=s]

s       V1
1: 1 31.14115
2: 2 29.40437
3: 3 30.20972
4: 4 29.91357

However, if my data.table command contain ifelse statement, I will not get similar table:

 d2<-d[,ifelse(a<b,"NA",sum(c)),by=s]
 d2


 s               V1
 1: 1               NA
 2: 1 31.1411493057385
 3: 1               NA
 4: 1               NA
 5: 1 31.1411493057385
 6: 2               NA
 7: 2               NA
 8: 2               NA
 9: 2               NA
10: 2               NA
11: 3               NA
12: 3               NA
13: 3               NA
14: 3 30.2097161230631
15: 3 30.2097161230631
16: 4               NA
17: 4               NA
18: 4               NA
19: 4               NA
20: 4 29.9135677714366

Is that possible to use the ifelse statement return a result just like the simple sum result table which return the unique non-na value under the each index value?

Thanks a lot!!!!!