Error using the Rdonlp2‏ Package

Dear experts,

I'm attempting to solve a constrained optimization problem using the Rdonlp2
package.

I created a Lagrange function (L=f(x)-lambda(g(x)-c)), where x is a vector
of 16 parameters. This is what I'm using as objective function in the code
below. In addition, I set bounds on these parameters (par.u and par.l). When
I run the code, I get the error message shown below. Any idea why or what
does it mean?

Thanks in advance for your help!

 ans <- donlp2(par=rate0, fn=L,par.u=par.u, par.l=par.l)

   1 fx=   2.869234e+08 upsi=  0.0e+00 b2n=  5.0e+08 umi=  0.0e+00 nr   0
si-1
   2 fx=  -2.111002e+09 upsi=  0.0e+00 b2n=  5.1e+08 umi=  0.0e+00 nr  14
si-1
   3 fx=  -5.961701e+09 upsi=  0.0e+00 b2n=  4.1e+08 umi=  0.0e+00 nr  16
si-1

*Error Message:*

Error in tryCatchList(expr, classes, parentenv, handlers) :
  SET_VECTOR_ELT() can only be applied to a 'list', not a 'character'

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