Error executing child request

[David C]

We have a user getting an error message as follows:

Error executing child request for handler 'ASP.frmpropertyclosings_aspx'.

The line in the code-behind page that is causing the error is below.

If Not Page.PreviousPage Is Nothing Then

I cannot figure out why this error is occurring and wondered if anybody had
any ideas. Below is the stack trace. Thanks.
-David

Stack Trace:
at System.Web.HttpServerUtility.ExecuteInternal(IHttpHandler handler,
TextWriter writer, Boolean preserveForm, Boolean setPreviousPage,
VirtualPath path, VirtualPath filePath, String physPath, Exception error,
String queryStringOverride)
at System.Web.UI.Page.get_PreviousPage()
at frmListingDocs.Page_Load(Object sender, EventArgs e) in
http://server//RFPWebOffice/frmListingDocs.aspx.vb:line 54
at System.Web.UI.Control.OnLoad(EventArgs e)
at System.Web.UI.Control.LoadRecursive()
at System.Web.UI.Page.ProcessRequestMain(Boolean
includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
at System.Web.UI.Page.ProcessRequest(Boolean
includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
at System.Web.UI.Page.ProcessRequest()
at System.Web.UI.Page.ProcessRequestWithNoAssert(HttpContext context)
at System.Web.UI.Page.ProcessRequest(HttpContext context)
at ASP.frmlistingdocs_aspx.ProcessRequest(HttpContext context)
at
System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
at System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean&
completedSynchronously)