how do you use IPropertySetStorage to get file properties?

See more:

C#

Here is some code I've inherited in my job:

public string GetProperty(SummaryPropId summaryPropertID)
{
    IPropertySetStorage pssSetStorage = null;
    Guid guidPropertySetStorage = new Guid("0000013A-0000-0000-C000-000000000046");
    Guid guidSummaryProperties = new Guid("F29F85E0-4FF9-1068-AB91-08002B27B3D9");
    IPropertyStorage psStorage = null;
    PropSpec[] pscSpecs = new PropSpec[1];
    PropVariant[] pvValues = new PropVariant[1];

    try
    {
        //Exit if file does not exist
        if (!fiSource.Exists) return null;
        // Open the file and its property set stream
        uint hresult = ole32.StgOpenStorageEx(fiSource.FullName, (int)(STGM.DIRECT_SWMR | STGM.READ | STGM.SHARE_DENY_NONE), (int)STGFMT.FILE, 0, System.IntPtr.Zero, System.IntPtr.Zero, ref guidPropertySetStorage, ref pssSetStorage);
        // Open the Summary Information property set
        int hresult2 = pssSetStorage.Open(ref guidSummaryProperties, (int)(STGM.DIRECT | STGM.SHARE_EXCLUSIVE), out psStorage);
        // Specify the property to be retrieved
        pscSpecs[0].ulKind = 1;
        pscSpecs[0].Name_Or_ID = new IntPtr((int)summaryPropertID);
        // Retrieve the value
        hresult2 = psStorage.ReadMultiple(1, pscSpecs, pvValues);
        // Release the Com objects
        Marshal.ReleaseComObject(psStorage);
        Marshal.ReleaseComObject(pssSetStorage);
        psStorage = null;
        pssSetStorage = null;
    }
    catch (Exception excException)
    {
        AuditMethodError(excException, System.Reflection.MethodInfo.GetCurrentMethod().DeclaringType.FullName + "." + System.Reflection.MethodBase.GetCurrentMethod().Name + " - " + excException.StackTrace);
    }
    finally
    {
        pssSetStorage = null;
        psStorage = null;
        pscSpecs = null;
        pvValues = null;
    }
    return Marshal.PtrToStringUni(pvValues[0].pointerValue);
}

The problem is that when it gets to the line int hresult2 = pssSetStorage.Open(ref guidSummaryProperties, (int)(STGM.DIRECT | STGM.SHARE_EXCLUSIVE), out psStorage) I'm receiving this exception:

excException = {"Object reference not set to an instance of an object."}

Here is my interface declaration:

[ComVisible(true), ComImport(), Guid("0000013A-0000-0000-C000-000000000046"), InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
public interface IPropertySetStorage
{
    uint Create([In] ref System.Guid rfmtid, [In] IntPtr pclsid, [In] int grfFlags, [In] int grfMode, ref IPropertyStorage propertyStorage);
    int Open([In] ref System.Guid rfmtid, [In] int grfMode, [Out] out IPropertyStorage propertyStorage);
}

Here is my STGM enumeration:

public enum STGM : int
{
    READ = 0x00000000,
    WRITE = 0x00000001,
    READWRITE = 0x00000002,
    SHARE_DENY_NONE = 0x00000040,
    SHARE_DENY_READ = 0x00000030,
    SHARE_DENY_WRITE = 0x00000020,
    SHARE_EXCLUSIVE = 0x00000010,
    PRIORITY = 0x00040000,
    CREATE = 0x00001000,
    CONVERT = 0x00020000,
    FAILIFTHERE = 0x00000000,
    DIRECT = 0x00000000,
    TRANSACTED = 0x00010000,
    NOSCRATCH = 0x00100000,
    NOSNAPSHOT = 0x00200000,
    SIMPLE = 0x08000000,
    DIRECT_SWMR = 0x00400000,
    DELETEONRELEASE = 0x04000000
}

Does anyone have any suggestions?