RSException on Matrix report rendering to PDF

I'm receiving the following exception (+stack trace) when trying to
render to PDF, but it works fine when rendering to all other formats.
Ideas?
~Lance
-- Error/Stack trace--
System.Web.Services.Protocols.SoapException:
System.Web.Services.Protocols.SoapException: Exception of type
Microsoft.ReportingServices.ReportRendering.ReportRenderingException
was thrown. -->
Microsoft.ReportingServices.Diagnostics.Utilities.RSException:
Exception of type
Microsoft.ReportingServices.ReportRendering.ReportRenderingException
was thrown. -->
Microsoft.ReportingServices.ReportProcessing.WrapperReportRenderingException:
Exception of type
Microsoft.ReportingServices.ReportRendering.ReportRenderingException
was thrown. -->
Microsoft.ReportingServices.ReportRendering.ReportRenderingException:
Exception of type
Microsoft.ReportingServices.ReportRendering.ReportRenderingException
was thrown. --> System.NullReferenceException: Object reference not
set to an instance of an object.
at
Microsoft.ReportingServices.Rendering.ImageRenderer.Matrix.FindHeaderRowsForVerticalResize(Int32
headerRowRow, Int32 level, Int32 lastRowForLevel, Int32& startRow,
Int32& endRow, Double& startTop)
at
Microsoft.ReportingServices.Rendering.ImageRenderer.Matrix.ResizeCellsVertically(ReAdjustments&
itemReAdjustment)
at
Microsoft.ReportingServices.Rendering.ImageRenderer.Matrix.CalculateItemMeasurement(Orientation
orientation, Double childrenMaximumDistance, ReAdjustments
itemReAdjustment, ItemContext savedContext)
at
Microsoft.ReportingServices.Rendering.ImageRenderer.RenderingItemContainer.CalculatePage(Orientation
orientation, Boolean repeatableParent, Boolean canDelete, Double&
minimumCoordinate, Double& distance)
at
Microsoft.ReportingServices.Rendering.ImageRenderer.RenderingItemContainer.CalculatePage(Orientation
orientation, Boolean repeatableParent, Boolean canDelete, Double&
minimumCoordinate, Double& distance)
at
Microsoft.ReportingServices.Rendering.ImageRenderer.RenderingItemContainer.CalculatePage(Orientation
orientation, Boolean repeatableParent, Boolean canDelete, Double&
minimumCoordinate, Double& distance)
at
Microsoft.ReportingServices.Rendering.ImageRenderer.ImageRendererBase.ProcessPages(CompositionBase
theComposition, Report theReport, Boolean needTotalPages, Int32
startPage, Int32 endPage, Boolean render)
at
Microsoft.ReportingServices.Rendering.ImageRenderer.PdfReport.RenderReport(Report
report, NameValueCollection deviceInfo, EvaluateHeaderFooterExpressions
evaluateHeaderFooterExpressions, CreateAndRegisterStream
createAndRegisterStreamCallback)
at
Microsoft.ReportingServices.Rendering.ImageRenderer.ImageRendererBase.Render(Report
report, NameValueCollection reportServerParameters, NameValueCollection
deviceInfo, NameValueCollection clientCapabilities,
EvaluateHeaderFooterExpressions evaluateHeaderFooterExpressions,
CreateAndRegisterStream createAndRegisterStream)
-- End of inner exception stack trace --
at
Microsoft.ReportingServices.Rendering.ImageRenderer.ImageRendererBase.Render(Report
report, NameValueCollection reportServerParameters, NameValueCollection
deviceInfo, NameValueCollection clientCapabilities,
EvaluateHeaderFooterExpressions evaluateHeaderFooterExpressions,
CreateAndRegisterStream createAndRegisterStream)
at
Microsoft.ReportingServices.ReportProcessing.ReportProcessing.a(DateTime
A_0, GetReportChunk A_1, ProcessingContext A_2, RenderingContext A_3,
CreateReportChunk A_4, Boolean& A_5)
-- End of inner exception stack trace --
at
Microsoft.ReportingServices.ReportProcessing.ReportProcessing.a(DateTime
A_0, GetReportChunk A_1, ProcessingContext A_2, RenderingContext A_3,
CreateReportChunk A_4, Boolean& A_5)
at
Microsoft.ReportingServices.ReportProcessing.ReportProcessing.RenderReport(DateTime
executionTimeStamp, GetReportChunk getCompiledDefinitionCallback,
ProcessingContext pc, RenderingContext rc)
at
Microsoft.ReportingServices.Library.RSService.RenderAsLive(CatalogItemContext
reportContext, ItemProperties properties, ParameterInfoCollection
effectiveParameters, Guid reportId, ClientRequest session, String
description, ReportSnapshot intermediateSnapshot,
DataSourceInfoCollection thisReportDataSources, Boolean
cachingRequested, Warning[]& warnings, ReportSnapshot&
resultSnapshotData, DateTime& executionDateTime,
RuntimeDataSourceInfoCollection& alldataSources, UserProfileState&
usedUserProfile)
at
Microsoft.ReportingServices.Library.RSService.RenderAsLiveOrSnapshot(CatalogItemContext
reportContext, ClientRequest session, Warning[]& warnings,
ParameterInfoCollection& effectiveParameters)
at
Microsoft.ReportingServices.Library.RSService.RenderFirst(CatalogItemContext
reportContext, ClientRequest session, Warning[]& warnings,
ParameterInfoCollection& effectiveParameters, String[]&
secondaryStreamNames)
at
Microsoft.ReportingServices.Library.RenderFirstCancelableStep.Execute()
at
Microsoft.ReportingServices.Diagnostics.CancelablePhaseBase.ExecuteWrapper()
-- End of inner exception stack trace --
at
Microsoft.ReportingServices.Diagnostics.CancelablePhaseBase.ExecuteWrapper()
at
Microsoft.ReportingServices.Library.RenderFirstCancelableStep.RenderFirst(RSService
rs, CatalogItemContext reportContext, ClientRequest session,
JobTypeEnum type, Warning[]& warnings, ParameterInfoCollection&
effectiveParameters, String[]& secondaryStreamNames)
at
Microsoft.ReportingServices.WebServer.ReportingService.Render(String
Report, String Format, String HistoryID, String DeviceInfo,
ParameterValue[] Parameters, DataSourceCredentials[] Credentials,
String ShowHideToggle, Byte[]& Result, String& Encoding, String&
MimeType, ParameterValue[]& ParametersUsed, Warning[]& Warnings,
String[]& StreamIds)
-- End of inner exception stack trace --
at
Microsoft.ReportingServices.WebServer.ReportingService.Render(String
Report, String Format, String HistoryID, String DeviceInfo,
ParameterValue[] Parameters, DataSourceCredentials[] Credentials,
String ShowHideToggle, Byte[]& Result, String& Encoding, String&
MimeType, ParameterValue[]& ParametersUsed, Warning[]& Warnings,
String[]& StreamIds)
at
System.Web.Services.Protocols.SoapHttpClientProtocol.ReadResponse(SoapClientMessage
message, WebResponse response, Stream responseStream, Boolean
asyncCall)
at
System.Web.Services.Protocols.SoapHttpClientProtocol.Invoke(String
methodName, Object[] parameters)Anybody have an idea/suggestion?