Report Builder 3.0 Determine value for 1 parameter based on the value selected in another parameter

In my report, the GRP parameter has the following labels and values: E5 E5 F5 F5 G1 G1 G2 G2I have a second parameter (TMP) that should be completed based on the value selected in the GRP parameter.I have the following expression to set the default value for the TMP parameter:=IIf(Parameters!GRP.Value ="E5", "A0485", IIf(Parameters!GRP.Value ="F5", "A0465", IIf(Parameters!GRP.Value ="G1", "A0141", IIf(Parameters!GRP.Value ="G2", "A0142", ""))))The TMP parameter will be internal and has no available values. What I'm trying to do is when the user selects "G1" in the GRP parameter, the TMP parameter should display "A0142".At this time, no matter what I select in the GRP parameter, the TMP parameter is blank. Any suggestions?Thanks for the help..........