急！请教VB.NET混合编程的问题！

用FPS4.0编写生成一个DLL文件“test.dll”（源代码如下）供编写的一个程序调用（源代码如下）。但运行总是出现<无法在DLL“XXXXX\test.dll”中找到名为‘XXX’的入口点>的错误。请高手帮忙啊！

FORTRAN程序：

SUBROUTINE calltest(X,Y,result)

! MS$ ATTRIBUTES ALIAS:"calltest"::calltest
! MS$ ATTRIBUTES DLLEXPORT::calltest

Integer*2 X
Real*4 Y,result

result=X*Y

END SUBROUTINE


程序：

Public Class CallFortran

    Declare Sub calltest Lib "test.dll" _
    (ByVal vbx As Integer, ByVal vby As Single, ByRef vbresult As Single)
-------------------------------------------------------------------

    Private Sub btnjisuan_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnjisuan.Click


        Dim vbx As Single
        Dim vbresult As Single
        Dim vby As Integer


        vbx = 4
        vby = 5

        Call calltest(vbx, vby, vbresult)

        lbljieguo.Text = vbresult


    End Sub
End Class