PDF Export issue in Reporting Services

I have come across an error when exporting a report to PDF in Reporting Services 2008.

The error message is;

Object reference not set to an
instance of an object. Description:
An unhandled exception occurred during
the execution of the current web
request. Please review the stack trace
for more information about the error
and where it originated in the code.

Exception Details: System.Exception:
Object reference not set to an
instance of an object.

Source Error:

An unhandled exception was generated
during the execution of the current
web request. Information regarding the
origin and location of the exception
can be identified using the exception
stack trace below.

Stack Trace:

[Exception: Object reference not set
to an instance of an object.]

[Exception: An error occurred during
rendering of the report.]

[Exception: An error occurred during
rendering of the report.]
Microsoft.Reporting.WebForms.ServerReport.ServerUrlRequest(Boolean
isAbortable, String url, Stream
outputStream, String& mimeType,
String& fileNameExtension) +520
Microsoft.Reporting.WebForms.ServerReport.InternalRender(Boolean
isAbortable, String format, String
deviceInfo, NameValueCollection
urlAccessParameters, Stream
reportStream, String& mimeType,
String& fileNameExtension) +963
Microsoft.Reporting.WebForms.ServerReport.Render(String
format, String deviceInfo,
NameValueCollection
urlAccessParameters, Stream
reportStream, String& mimeType,
String& fileNameExtension) +28
Microsoft.Reporting.WebForms.ServerModeSession.RenderReport(String
format, Boolean
allowInternalRenderers, String
deviceInfo, NameValueCollection
additionalParams, Boolean
cacheSecondaryStreamsForHtml, String&
mimeType, String& fileExtension) +85
Microsoft.Reporting.WebForms.ExportOperation.PerformOperation(NameValueCollection
urlQuery, HttpResponse response) +150
Microsoft.Reporting.WebForms.HttpHandler.ProcessRequest(HttpContext
context) +183
System.Web.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
+181 System.Web.HttpApplication.ExecuteStep(IExecutionStep
step, Boolean& completedSynchronously)
+75

However this error doesn't always appear on this report, it has only occurred on several occasions, dependent on the dates we use in the parameters of the report. I am new to Reporting Services so could anyone help me figure out what could be causing the problem?