nls() error: Object not found

Dear R-helpers;

	Using nls() to fit a function,Rdum, defined below I stumbled on an
error: "Error in eval(expr, envir, enclos): Object "s0" not found." 

	The function Rdum is defined as

     Rdum <- deriv(~ h1 * (s0 + sl0*sl + sm0*sm  + sp01*sp1 + sp02*sp2 +
sp03*sp3+sp04*sp4) *
      ((1 - exp(-(s1 + sl1*sl + sm1*sm  + sp11*sp1 + sp12*sp2  +
sp14*sp4)*t2)) /
       (1 - exp(-(s1 + sl1*sl + sm1*sm  + sp11*sp1 + sp12*sp2 +
sp14*sp4)*t1)))^
       (s2 + sl2*sl+sm2*sm+ sp21*sp1+sp22*sp2+sp23*sp3+sp24*sp4),
  c("s0", "s10","sm0", "sp01","sp02","sp03","sp04",
     "s1", "sl1","sm1", "sp11", "sp12","sp14",
     "s2","sl2","sm2", "sp21","sp22","sp23","sp24"),
     function(s0,sl0,sm0,sp01,sp02,sp03,sp04, s1, sl1, sm1, sp11,sp12, sp14,

    s2, sl2, sm2, sp21,sp22, sp23, sp24, h1, t1,t2, sl, sm,sp1, sp2, sp3,
sp4){})

and variables t1, t2, h1, h2, sl, sm, sp1, sp2, sp3, and sp4 in the gbht10D
data frame are fitted to the function by

   Dum.nls <- nls(h2 ~ Rdum(s0, sl0, sm0, sp01, sp02, sp03, sp04, sl,
sl1,sm1,sp11,sp12, sp14,
   s2, sl2, sm2, sp21, sp22, sp23, sp24,h1, t1, t2, sl, sm, sp1, sp2, sp3,
sp4), data=gbht10D,
   start=c(0.8413,-0.1602,-0.0156,0.0527,0.0513,0.00314,0.0378,
   -0.00598,-0.0125,0.00328,0.00989,0.0113,0.00583,
   1.9255,0.9427,0.2408,-0.0643,0.0047,0.0016,-0.00519))

    The complained object "s0" is one of the 20 parameters in the function
to be estimated from the dataset gbht10D. 

    The nls() script is relatively short. Stepping into the source code, I
located the offending line:

Browse[1]> start
 [1]  0.84130 -0.16020 -0.01560  0.05270  0.05130  0.00314  0.03780 -0.00598
-0.01250
[10]  0.00328  0.00989  0.01130  0.00583  1.92550  0.94270  0.24080 -0.06430
0.00470
[19]  0.00160 -0.00519
Browse[1]> n
debug: pnames <- names(start)
Browse[1]> n
debug: varNames <- varNames[is.na(match(varNames, pnames, nomatch = NA))]
Browse[1]> n
debug: varIndex <- sapply(varNames, function(varName, data, respLength) {
    length(eval(as.name(varName), data))%%respLength == 0
}, data, length(eval(formula[[2]], data)))
Browse[1]> varNames
 [1] "h2"   "s0"   "sl0"  "sm0"  "sp01" "sp02" "sp03" "sp04" "sl"   "sl1"
"sm1"  "sp11"
[13] "sp12" "sp14" "s2"   "sl2"  "sm2"  "sp21" "sp22" "sp23" "sp24" "h1"
"t1"   "t2"  
[25] "sm"   "sp1"  "sp2"  "sp3"  "sp4" 
Browse[1]> n
Error in eval(expr, envir, enclos) : Object "s0" not found. 

The "s0" is shown in varNames. Is the error message misleading? Or did I
miss something? Any iedas and suggestions?

I am using R 2.0.0 on Windows XP.

TIA,

Richard Yang