MS Access VBA: Can not transfer array variables to user-defined function

Question

This is the first time for me to write VBA code involving arrays. I have a problem that MatrixMult function (which calculates M1 and M2 matrices) can not be called from MatrixText function. The problem is in "MOut = MatrixMult(M1, M2)" line.

When I compile the module, the "M1" in that line is highlighted and the message is, "Compile error: Type mismatch: array or user-defined type expected." Please, anybody can help me?

Option Compare Database
Option Explicit
Public Function MatrixText() As Double()
    Dim M1(), M2(), MOut() As Double
    Dim Row1, Col1, Row2, Col2, Iter1, Iter2 As Integer
    Rem
    Row1 = 5
    Col1 = 16
    Row2 = 16
    Col2 = 5
    ReDim MOut(1 To Row1, 1 To Col2)
    ReDim M1(1 To Row1, 1 To Col1)
    ReDim M2(1 To Row2, 1 To Col2)
    Rem
    For Iter1 = 1 To Row1
        For Iter2 = 1 To Col1
            M1(Iter1, Iter2) = Rnd
            M2(Iter2, Iter1) = Rnd
        Next Iter2
    Next Iter1
    MOut = MatrixMult(M1, M2)
    MatrixText = MOut
End Function
Public Function MatrixMult(ByRef mA() As Double, ByRef mB() As Double) As Double()
    Dim ARow, ACol, BRow, Bcol, Iter1, Iter2, Iter3 As Integer
    Dim MOutput() As Double
    Rem
    ARow = UBound(mA, 1)
    ACol = UBound(mA, 2)
    BRow = UBound(mB, 1)
    Bcol = UBound(mB, 2)
    If ACol = BRow Then
        ReDim MOutput(1 To ARow, 1 To Bcol)
        For Iter1 = 1 To ARow
            For Iter2 = 1 To Bcol
                For Iter3 = 1 To ACol
                    MOutput(Iter1, Iter2) = MOutput(Iter1, Iter2) + mA(Iter1, Iter3) * mB(Iter3, Iter2)
                Next Iter3
            Next Iter2
        Next Iter1
        MatrixMult = MOutput
    Else
        Rem the size of mA and mB do not match !!!!!!!!!!!!!!!
        Rem MatrixMult() = 0
    End If
End Function

Answer 1

This line does not do what you expect:

Dim M1(), M2(), MOut() As Double

In fact it does:

Dim M1() As Variant, M2() As Variant, MOut() As Double

You must add the data type to each variable.

Dim multiple objects as Integer / Variant / etc.?