I do not know how your macro looks, but basically, there is a difference between InputBox and Application.InputBox. (I guess you use the first one )
With the last one, you can select the type of data permitted for an entry:
Type:=0 A formula
Type:=1 A number
Type:=2 Text (a string)
Type:=4 A logical value (True or False)
Type:=8 A cell reference, as a Range object
Type:=16 An error value, such as #N/A
Type:=64 An array of values
And, as you can see, you can enter a default value if you need that...