I am getting an error exporting a report to pdf format from an ASP.NET page. The error is

Error in File C:\WINDOWS\TEMP\temp_c4ed118d-8c8f-4ef1-89a3-42d658fc2576.rpt: Error detected by export DLL:

the line that generates the error is

rpt.Export();

Another page generates a pdf fine with similar code.

Any ideas?