Visual Basic - Error al convertir imagen almacenad en byte para mostrar en PictureBox

Dim sql As String = "SELECT foto from estudiante where identidad_alumno=:identidad"

Dim query As NpgsqlCommand = New NpgsqlCommand(sql, cnn)

query.Parameters.Add(New NpgsqlParameter("identidad", NpgsqlDbType.Varchar)).Value = Me.Txtbusqueda.Text

Dim adapter As NpgsqlDataAdapter = New NpgsqlDataAdapter(query)

Dim ds As New DataSet()

adapter.Fill(ds, "estudiante")

Dim c As Integer = ds.Tables("estudiante").Rows.Count

MessageBox.Show(c)

If c > 0 Then

	Dim bytblod() As Byte = ds.Tables("estudiante").Rows(c - 1)("foto")

	Dim stmblodata As New MemoryStream(bytblod)

	Me.pbmostrar.Image = Image.FromStream(stmblodata)

End If