Change the "Prompting Text" of a crystal report parameter using Visual Basic 6.0

[canu]

Hello,
I would like to change the "Prompting Text" of a crystal report parameter.
If I consider the dbobject as View then I am able to do this with the following line

CrRep.ParameterFields(1).Prompt = "MyString"

PROBLEM:
If the Parameter is a input field of a stored procedure, and i try to do the same:
CrRep.ParameterFields(1).Prompt = "MyString"

I am receiving the following error:
"Not Report Parameter."
Error Number: -2147189575 (80047cb9)

Is there any solution for this problem?