Unable to put open dataset into define mode

Hello,

I am trying to put a dataset into define mode but I
get an error status of -39.  I can't find where the
error codes are defined in the documentation so I'm
pretty clueless as to why this is happening.  I am
using the NetCDF v3.0 C interface in a C++ program. 
The relevant code is included below.  Thanks for any
suggestions as to how to get around this.


-James


--------------------------------------


/*****************************************
* Write a grid to an NetCDF file.
*
*@param path the path to the NetCDF file
*******************************************/
int GeoGrid::writeGridToNetCDF (char *path)
{
        // IDs used throughout the method
        int status;
        int ncid;


        // open the NetCDF
        status = nc_create(path, NC_NOCLOBBER, &ncid);
        if (status != NC_NOERR)
        {
                return 1;
        }


        // put into define mode
        status = nc_redef(ncid);
        if (status != NC_NOERR)
        {
                // it is here that I get status = -39
                return 2;
        }



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