input string was not in a correct format.

Hello, I've read other messages on this error and I think that my code looks correct (obviously it's not). Can someone see if they can see my error?

from the SP:
CREATE PROCEDURE dbo.EmpInfoInsert (
@NamePfx VARCHAR(8) = NULL
, @FirstName VARCHAR(50)
, @MI CHAR(1)
, @LastName VARCHAR(50)
, @NameSfx VARCHAR(8) = NULL
, @PrimaryID VARCHAR(9)
, @Sex CHAR(1)
, @BirthDate SMALLDATETIME
, @Address VARCHAR(128)
, @Address2 VARCHAR(128) = NULL
, @City VARCHAR(50)
, @State TINYINT
, @ZipCode VARCHAR(10)
, @MarStatus TINYINT
, @HomePhone VARCHAR(50) = NULL
, @BusPhone VARCHAR(50) = NULL
, @Ext VARCHAR(10) = NULL
, @Email VARCHAR(128)
, @AlternateID VARCHAR(50) = NULL
, @Dept VARCHAR(32) = NULL
, @JobDesc VARCHAR(256) = NULL
, @DateHired SMALLDATETIME = NULL
, @HoursPerWeek NUMERIC(5, 2)
, @PayPeriod TINYINT
, @DedPeriod TINYINT
, @GrossSalary MONEY
, @StateMarStatus VARCHAR(2)
, @StateExempt TINYINT
, @WitState TINYINT
, @FedMarStatus CHAR(1)
, @FedExempt TINYINT
, @ExtraTax SMALLMONEY = NULL
)
------------------------------
from the C# code (web service):
SqlParameter[] parms = {
new SqlParameter("@NamePfx", SqlDbType.VarChar, 8),
new SqlParameter("@FirstName", SqlDbType.VarChar, 50),
new SqlParameter("@MI", SqlDbType.Char, 1),
new SqlParameter("@LastName", SqlDbType.VarChar, 50),
new SqlParameter("@NameSfx", SqlDbType.VarChar, 8),
new SqlParameter("@PrimaryID", SqlDbType.VarChar, 9),
new SqlParameter("@Sex", SqlDbType.Char, 1),
new SqlParameter("@BirthDate", SqlDbType.SmallDateTime),
new SqlParameter("@Address", SqlDbType.VarChar, 128),
new SqlParameter("@Address2", SqlDbType.VarChar, 128),
new SqlParameter("@City", SqlDbType.VarChar, 50),
new SqlParameter("@State", SqlDbType.TinyInt),
new SqlParameter("@ZipCode", SqlDbType.VarChar, 10),
new SqlParameter("@MarStatus", SqlDbType.TinyInt),
new SqlParameter("@HomePhone", SqlDbType.VarChar, 50),
new SqlParameter("@BusPhone", SqlDbType.VarChar, 50),
new SqlParameter("@Ext", SqlDbType.VarChar, 10),
new SqlParameter("@Email", SqlDbType.VarChar, 128),
new SqlParameter("@AlternateID", SqlDbType.VarChar, 50),
new SqlParameter("@Dept", SqlDbType.VarChar, 32),
new SqlParameter("@JobDesc", SqlDbType.VarChar, 256),
new SqlParameter("@DateHired", SqlDbType.SmallDateTime),
new SqlParameter("@HoursPerWeek", SqlDbType.Float),
new SqlParameter("@PayPeriod", SqlDbType.TinyInt),
new SqlParameter("@DedPeriod", SqlDbType.TinyInt),
new SqlParameter("@GrossSalary", SqlDbType.Money),
new SqlParameter("@StateMarStatus", SqlDbType.VarChar, 2),
new SqlParameter("@StateExempt", SqlDbType.TinyInt),
new SqlParameter("@WitState", SqlDbType.TinyInt),
new SqlParameter("@FedMarStatus", SqlDbType.Char, 1),
new SqlParameter("@FedExempt", SqlDbType.TinyInt),
new SqlParameter("@ExtraTax", SqlDbType.SmallMoney)
};

parms[0].Value = namePfx == string.Empty? Convert.DBNull : namePfx;
parms[1].Value = firstName;
parms[2].Value = mi;
parms[3].Value = lastName;
parms[4].Value = nameSfx == string.Empty? Convert.DBNull : nameSfx;
parms[5].Value = primaryID;
parms[6].Value = sex;
parms[7].Value = birthDate;
parms[8].Value = address;
parms[9].Value = address2;
parms[10].Value = city;
parms[11].Value = state;
parms[12].Value = zipCode;
parms[13].Value = marStatus;
parms[14].Value = homePhone == string.Empty? Convert.DBNull : homePhone;
parms[15].Value = busPhone == string.Empty? Convert.DBNull : busPhone;
parms[16].Value = ext == string.Empty? Convert.DBNull : ext;
parms[17].Value = email;
parms[18].Value = alternateID == string.Empty? Convert.DBNull : alternateID;
parms[19].Value = dept == string.Empty? Convert.DBNull : dept;
parms[20].Value = jobDesc == string.Empty? Convert.DBNull : jobDesc;
parms[21].Value = dateHired == DateTime.MinValue? Convert.DBNull : dateHired;
parms[22].Value = hoursPerWeek;
parms[23].Value = payPeriod;
parms[24].Value = dedPeriod;
parms[25].Value = grossSalary;
parms[26].Value = stateMarStatus;
parms[27].Value = stateExempt;
parms[28].Value = witState;
parms[29].Value = fedMarStatus;
parms[30].Value = fedExempt;
parms[31].Value = extraTax == string.Empty? Convert.DBNull : extraTax;

SqlHelper.ExecuteNonQuery(_transaction, CommandType.StoredProcedure, "EmpInfoInsert", parms);
This call gives the error. I've looked and all of my parms match the datatype of the sproc. I've stepped through to make sure that all values assigned are what I expect them to be.
Please Help...

Thanks, Dan