how to estimate parameters for groups using nlslist function of r?

Question

I want to fit the curve and estimate the parameters (a, b1 and b2) for each Id group using the following model:
y= a + b1*exp^-0.05*dim + b2*dim
the sample of my data file looks as follow (wt.txt)

 Id wt. dim   
 4 546.5 1     
 4 542.5 2   
 4 539 3   
 4 535 4   
 10 488 1   
 10 490.5 2   
 10 470.5 3   
 10 472 4   

here is the model i used and error message i got:

 >m1<- nlsList(wt ~ a + b1*exp^-0.05*dim + b2*dim | Id, data = wt.txt)
 error :there were 50 or more warnings (use warnings (use warnings () to see       the first 50)

do you help me fitting the model appropriately?