Basic Powerpivot question

[ERC2]

I imported a table from excel that has text and numbers as a linked table into powerpivot. I get the following error message when I tried to create a pivotchart. The column header was "grade" this time but I have tried it with many different columns with the same error.
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Error Message:
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Could not add the field "grade" to the PivotTable. Please ensure the field exists and is calculated, and try refreshing the PivotTable.
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Call Stack:
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at Microsoft.AnalysisServices.Modeler.FieldList.ExcelInterOpUtil.AddToDataFields(ICalculatedMember calculatedMember, Int32 positionIndex, Boolean isSpecialColumnBasedNamedSetPresent)
at Microsoft.AnalysisServices.Modeler.FieldList.GeminiPivot.SetFieldOrientation(FieldLocation location, IGeminiColumn column, Int32 positionIndex)
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at Microsoft.AnalysisServices.Modeler.FieldList.GeminiPivot.SetFieldOrientation(FieldLocation location, IGeminiColumn column, Int32 positionIndex)
at Microsoft.AnalysisServices.Modeler.FieldList.GeminiPivot.AddToFieldList(FieldLocation location, IGeminiColumn column, Int32 positionIndex)
at Microsoft.AnalysisServices.Modeler.FieldList.GeminiPivot.MoveField(IGeminiColumn column, FieldLocation source, FieldLocation target, Int32 positionIndex)
at Microsoft.AnalysisServices.Modeler.FieldList.GeminiPivot.AddField(IGeminiColumn column, Int32 index)
at Microsoft.AnalysisServices.Modeler.FieldList.FieldListControl.fieldsTreeView_AfterCheck(Object sender, TreeViewEventArgs e)
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