Identical Split() call works fine in one program but not in another

[coffent]

I have a test line in a VB program:

Dim testArray() As String = Split("ab\c\d", "\")

which I copied more or less straight from a Visual Basic reference.

It works fine in one program, but in another I get the error message:

Error BC30519 Overload resolution failed because no accessible 'Split' can be called without a narrowing conversion:
'Public Overloads Function Split(ParamArray separator As Char()) As String()': Argument matching parameter 'separator' narrows from 'String' to 'Char'.
'Public Overloads Function Split(ParamArray separator As Char()) As String()': Argument matching parameter 'separator' narrows from 'String' to 'Char'.
'Public Overloads Function Split(separator As Char(), count As Integer) As String()': Argument matching parameter 'separator' narrows from 'String' to 'Char()'.
'Public Overloads Function Split(separator As Char(), count As Integer) As String()': Argument matching parameter 'count' narrows from 'String' to 'Integer'.
'Public Overloads Function Split(separator As Char(), options As StringSplitOptions) As String()': Argument matching parameter 'separator' narrows from 'String' to 'Char()'.
'Public Overloads Function Split(separator As Char(), options As StringSplitOptions) As String()': Argument matching parameter 'options' narrows from 'String' to 'StringSplitOptions'.

I've tried any number of different ways of implementing the Split, but nothing works.

I also tried:

Dim testString As String = "Look at these!"
'Returns an array containing "Look", "at", and "these!".
Dim testArray() As String = Split(testString)

again copied directly from a Visual Basic reference. It gives the error message:

Error BC30469 Reference to a non-shared member requires an object reference.

Again, the same statements work fine when run in another program.

FWIW, I have:

Imports System.Threading
Imports System.IO
Imports System.String

Is there a reference missing, or something corrupt?