Vba bitness migration - Type Mismatch on "-" operator

We have recently begun migrating to Office 365. First day after migration, a user reported a problem with the following piece of VBA code, using the kernel32.dll library:

'@Esposed
'@Folder System.Text
Option Explicit

Private Declare PtrSafe Sub CopyMemory Lib "kernel32.dll" Alias "RtlMoveMemory" (Byaval dst As Long, ByVal src As Long, ByVal length as Long) 

Private Const DEFAULT_CHARACTER_CAPACITY As Long = &H10
Pivate CurrentByteLength As Long
Private StringBuffer() As Byte

Private Sub Class_Initialize() 
    ReDim StringBuffer(0 To (DEFAULT_CHARACTER_CAPACITY * 2) - 1) 
                                                            '^ Type Mismatch!!!                             

End Sub

It turns out that our previous version of Office(2016) was 32bit, so we ended up with 2 migrations in one.

I have tried changing all the Long references to LongPtr, and converting all the constants using the CLngPtr function, with no success. Any idea on what is going on here?