[R] Help me about singular error nls

[jarod_v6]

Dear List

I have that problem:
this is file : PBS.txt
Time	RFU
0.0000	27.3021
0.0800	26.1565
0.1600	25.4836
0.2400	25.2104
0.3200	24.7967
0.4000	24.6310
0.4800	24.3472
0.5600	24.2905
0.6400	24.1394
0.7200	24.0244
0.8000	23.8532
0.8800	23.7908
0.9600	23.6668
1.0400	23.7458
1.1200	23.6787
1.2000	23.5651
1.2800	23.4757
1.3600	23.5172
1.4400	23.4748
1.5200	23.6037
1.6000	23.4516
1.6800	23.5655
1.7600	23.4365
1.8400	23.3553
1.9200	23.5021
2.0000	23.4123 


rm(list=ls())
print("ls()")
#carica Dati con file di testo

b <-read.table("PBS.txt", sep="\t", header= TRUE)
print("b")
b
nlmod1 <- nls(RFU ~ A + (B+c*Time)/(1+exp(k*(e-Time))),
 data = b,
start = list(A = 1, c = 1, B = 1, k =1, e = 1),
 trace = TRUE))

#-------------------------------------------------------------------------------------
13413478 :  1 1 1 1 1 
Errore in nls(RFU ~ A + (B + c * Time)/(1 + exp(k * (e - Time))), data = b,  : 
  gradiente singolare

Could you help me? The plot of data I don`t found errors and I don`t know  why sometimes in similar data function and sometimes not.
thanks for the help!