nlme vs nlmer

Hi

can you give me a hint how to convert a nlme(..) like

nlme(Y ~ fun(X,a,b,c),
     fixed  = list(a ~ 1, b ~ 1, c ~ 1),
     random = list(a ~ 1, b ~ 1, c ~ 1),
     groups = ~ location,
     data   = measurements,
     start  = c(a = 350, b = 0.2, c = 120))

to an equivalent nlmer(...)? The following seems to be wrong

nlmer(Y ~ fun(X,a,b,c) ~ (a  + b + c | location),
              data = measurements,
              start = c(a = 350, b = 0.2, c = 120))

it throws "gradient attribute of evaluated model must be a numeric matrix"

thx
Christof