exponential fit singularity in r - r

Question

I'm trying to fit a bi-exponential function on this dataset but I can't seem to get it to converge. I've tried using nls2 grid search for the best starting point, I've also tried using nlsLM but neither method works. Does anyone have any suggestions?
function: y = a1*exp(-n1*t) + a2*exp(-n2*t) + c
here is the code:
y <- c(1324,1115,1140,934,1013,982,1048,1143,754,906,895,900,765,808,680,731,728,794,706,531,629,629,519,514,516,454,465,630,415,347,257,363,275,379,329,263,301,315,283,354,230,257,196,268,262,236,220,239,255,213,275,273,294,169,257,178,207,169,169,297,
227,189,214,168,263,227,185,220,169,229,174,231,178,141,195,223,258,206,181,200,150,200,169,194,230,162,174,194,225,216,196,213,150,235,231,224,244,161,219,222,210,
186,188,197,177,251,248,223,273,145,257,236,214,194,211,213,175,168,223,192,318,
263,234,163,202,239,189,216,206,185,185,191,340,145,188,305,112,252,213,245,240,196,196,179,235,241,177,196,191,181,240,164,202,201,306,214,212,185,192,178,203,203,239,141,203,190,216,174,219,153,177,223,207,186,213,173,210,191,258,277)
t <- seq(1,length(y),1)
mydata <- data.frame(t=t,y=y)
library(nls2)
fo <- y~a1*exp(-n1*t)+a2*exp(-n2*t)+c
grd <- expand.grid(a1=seq(-12030,1100,by=3000),
a2=seq(-22110,1900,by=2000),
n1=seq(0.01,.95,by=0.4),
n2=seq(0.02,.9,by=0.25),
c=seq(100,400,by=50))
fit <- nls2(fo, data=allout, start=grd, algorithm='brute-force', control=list(maxiter=100))
fit2 <- nls(fo, data=allout, start=as.list(coef(fit)), control=list(minFactor=1e-12, maxiter=200),trace=F)
error: maximum iteration exceeded
However, if I use nlsLM then I get singularity gradient matrix at initial parameter estimate.