Why Attempt to read or write in protected memory.?

Question

What I’m doing is taking a frame of some videos, but whenever I try to access the saved images I get Read or write attempt in protected memory. I can’t use them after "saving". What I’m doing wrong?

Imports System.Drawing   
Imports System.Windows.Forms  
Imports Emgu.CV

    Private Const LocFrame = 0.2
    Public Shared listMiniatura As New ImageList With {.ColorDepth = ColorDepth.Depth32Bit, .ImageSize = New Size(255, 255)}        

    Public Shared Sub GetThumbnail(path As String)
            If listMiniatura.Images.ContainsKey(path) Then
                Exit Sub
            End If

            Dim f As Integer
            Dim m As Mat

            'Abre o video para pegar o frame
            Using cap As New VideoCapture(path)
                If cap.IsOpened Then

                    'Posição do frame que será retirado
                    f = cap.GetCaptureProperty(CvEnum.CapProp.FrameCount) * LocFrame
                    cap.SetCaptureProperty(CvEnum.CapProp.PosFrames, f)
                    m = New Mat
                    cap.Read(m)

                    'Salva em listMiniatura
                    listMiniatura.Images.Add(path, m.Bitmap)                    
                End If
            End Using
    End Sub