No, it's not case sensitive.
OP: Only thing I can think of is one of the following:
1) You're trying to set a variable of none INT datatype to the UBound value
e.g.
Dim a AS String
a=UBound(YourArray)
2) The 'array' your setting in the UBound isn't in fact an array.
e.g.
Dim YourArray as String
Dim A As Int
A=UBound(YourArray)
As a dead easy example, the following works for me.
Dim MyArray = Array(1,2,3,4,5)
MsgBox(UBound(MyArray))
Shows a message box on the screen of 5
MsgBox(UBound(MyArray)-1)
Shows a message box on the screen of 4.