Adding Custom Properties to a Drawing

I am not the best programmer so I'm hoping this is a simple solution for those who are more experienced, but I am trying to write a macro that will add a custom property to a drawing. 

When I run the below code, I get the error "Object doesn't support this property or method"... and it highlights the line

                     Set swCustPropMgr = Part.CustomPropertyManager

It seems it has a problem with the CustomPropertyManager?

Here is my code below... just wondering if anyone could help me?

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Private Sub CommandButton1_Click()

Dim swCustPropMgr As SldWorks.CustomPropertyManager

Dim bRet As String

Set swApp = Application.SldWorks

Set Part = swApp.ActiveDoc

Set swCustPropMgr = Part.CustomPropertyManager

bRet = swCustPropMgr.Add2("Description", swCustomInfoText, "blah blah")

End Sub

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Thanks in advance,

Brett

SolidworksApi macros