"Application uses a value of the wrong type" Error

The following is the code that I am using:
Stored Procedure (Partial code):

CREATE  PROCEDURE GetRespondentDemographics
      @Email nvarchar(75) = null,
      @Respondentid dec(19,0) = null

.......
 
My ASP Code:
<%
      Dim ConnectionString
      Dim ofso
      Dim otmp

        ConnectionString="Provider=SQLOLEDB;Data Source=TEST_W2K_SQL2K;Database=harrispoll;UID=paramtestuser;PWD=paramtestuser;Network Library=dbmssocn;"

      Dim objconn, objRS, adocmd
    Set objconn = Server.CreateObject("ADODB.Connection")
    objconn.Open (ConnectionString)

    Set adocmd = Server.CreateObject("ADODB.Command")
    adocmd.CommandText = "GetRespondentDemographics"
    adocmd.CommandType = adCmdStoredProc
 
    objRS = Server.CreateObject("ADODB.Recordset")
    adocmd.activeConnection = objconn
    adocmd.Parameters.Append adocmd.CreateParameter("@Email",adVarChar, adParamInput, 75)
    adocmd.Parameters.Append adocmd.CreateParameter("@Respondentid",adDecimal, adParamInput, 19)
    adocmd.Parameters("@Respondentid").Precision = 19
    adocmd.Parameters("@Respondentid").NumericScale = 0
    adocmd.Parameters("@Email") = Request.Form("email")
    adocmd.Parameters("@Respondentid") = Request.Form("respondentid")
    set objRS = adocmd.Execute

Ok. I have 2 fields (RespondentID and Email). The user must select at least one of the fields even though both fields can be filled out. Now my problem is that when I only enter the respondentID, I am able to get the results back without a problem BUT when I only enter the email field, I get the following error:
"Error Type:
ADODB.Command (0x800A0D5D)
Application uses a value of the wrong type for the current operation.
/param/HPoll_email_respondent_results.asp, line 28"

The line this is referring to is: adocmd.Parameters("@Respondentid") = Request.Form("respondentid")
When I comment out this field and I go back and enter an email within the email field, the program executes and displays the results without a problem. I do not understand why I am getting this error. The line that it is referring to as incorrect appears correct from my point of view. Any suggestions as to why I might getting this error? Thanks