GradientFill, but it comes up black

[penagate]

Trying to use GradientFill to paint a picturebox with a gradient from left to right, but it all comes up black.

Code:

Public Sub PaintMe( _
    ByVal x As Long, _
    ByVal y As Long, _
    Optional ByVal pfForceRepaint As Boolean = False _
)
'...
Dim lStartColour        As Long
Dim lEndColour          As Long
Dim tvxVertices(1)      As TRIVERTEX
Dim udtGradRect         As GRADIENT_RECT
 
    ' ...
 
        ' paint the background gradient
        lStartColour = GradientColorStart
        lEndColour = GradientColorEnd
 
        RGBToTriplex lStartColour, VarPtr(tvxVertices(0).Red), 2
 
        With tvxVertices(1)
            RGBToTriplex lEndColour, VarPtr(.Red), 2
 
            .x = picBkgd.ScaleWidth
            .y = picBkgd.ScaleHeight
        End With
 
        udtGradRect.LowerRight = 1
 
        GradientFill picBkgd.hDC, _
                     tvxVertices(0), _
                     2, _
                     udtGradRect, _
                     1, _
                     menmGradientDirection
 
    ' ...
 
End Sub

In case you're wondering (and this bit works):

Sub RGBToTriplex( _
    ByVal lRGB As Long, _
    ByVal lOutAddress As Long, _
    ByVal cbComponentSize As Long _
)
    RtlMoveMemory ByVal lOutAddress, ByVal VarPtr(lRGB), 1
    RtlMoveMemory ByVal lOutAddress + cbComponentSize, ByVal VarPtr(lRGB) + 1, 1
    RtlMoveMemory ByVal lOutAddress + 2 * cbComponentSize, ByVal VarPtr(lRGB) + 2, 1
End Sub

The snipped sections are unrelated to the drawing itself.

Anyone spot anything from the API call that I've missed?