Syntax Error When Building Up A Where Clause In Stored Procedure

Can anyone tell me why the line highlighted in blue produces the following error when I try to run this stored proc? I know the parameters are set properly as I can see them when debugging the SP.

I'm using this type of approach as my application is using the objectdatasource with paging. I have a similar SP that doesn't have the CategoryId and PersonTypeId parameters and that works fine so it is the addition of these new params that has messed up the building of the WHERE clause

The Error is:
"Syntax error converting the varchar value '  WHERE CategoryId = ' to a column of data type int."

Thanks

Neil

CREATE PROCEDURE dbo.GetPersonsByCategoryAndTypeByName (@CategoryId int, @PersonTypeId int, @FirstName varchar(50)=NULL, @FamilyName varchar(50)=NULL, @StartRow int, @PageSize int)

AS

Declare @WhereClause varchar(2000)
Declare @OrderByClause varchar(255)
Declare @SelectClause varchar(2000)

CREATE TABLE #tblPersons (
 ID int IDENTITY PRIMARY KEY ,
 PersonId int ,
 TitleId int NULL ,
 FirstName varchar (50)  NULL ,
 FamilyName varchar (50)  NOT NULL ,
 FullName varchar (120)  NOT NULL ,
 AltFamilyName varchar (50)  NULL ,
 Sex varchar (6)  NULL ,
 DateOfBirth datetime NULL ,
 Age int NULL ,
 DateOfDeath datetime NULL ,
 CauseOfDeathId int NULL ,
 Height int NULL ,
 Weight int NULL ,
 ABO varchar (3)  NULL ,
 RhD varchar (8)  NULL ,
 Comments varchar (2000)  NULL ,
 LocalIdNo varchar (20)  NULL ,
 NHSNo varchar (10) NULL ,
 CHINo varchar (10)  NULL ,
 HospitalId int NULL ,
 HospitalNo varchar (20)  NULL ,
 AltHospitalId int NULL ,
 AltHospitalNo varchar (20)  NULL ,
 EthnicGroupId int NULL ,
 CitizenshipId int NULL ,
 NHSEntitlement bit NULL ,
 HomePhoneNo varchar (12)  NULL ,
 WorkPhoneNo varchar (12)  NULL ,
 MobilePhoneNo varchar (12)  NULL ,
 CreatedBy varchar(40) NULL ,
 DateCreated smalldatetime NULL ,
 UpdatedBy varchar(40) NULL ,
 DateLastUpdated smalldatetime NULL,
 UpdateId int
)

SELECT @OrderByClause = ' ORDER BY FamilyName, FirstName'

SELECT @WhereClause = '  WHERE CategoryId = ' +  @CategoryId + ' AND PersonTypeId = ' + @PersonTypeId

If NOT @Firstname IS NULL
BEGIN
 SELECT @WhereClause = @WhereClause + ' AND FirstName LIKE ISNULL(''%'+ @FirstName + '%'','''')'
END

If NOT @FamilyName IS NULL
BEGIN
 SELECT @WhereClause = @WhereClause + ' AND (FamilyName LIKE ISNULL(''%'+ @FamilyName + '%'','''') OR AltFamilyName LIKE ISNULL(''%'+ @FamilyName + '%'',''''))'
END

Select @SelectClause = '
INSERT INTO #tblPersons
(
 PersonId,
 TitleId,
 FirstName,
 FamilyName ,
 FullName,
 AltFamilyName,
 Sex,
 DateOfBirth,
 Age,
 DateOfDeath,
 CauseOfDeathId,
 Height,
 Weight,
 ABO,
 RhD,
 Comments,
 LocalIdNo,
 NHSNo,
 CHINo,
 HospitalId,
 HospitalNo,
 AltHospitalId,
 AltHospitalNo,
 EthnicGroupId,
 CitizenshipId,
 NHSEntitlement,
 HomePhoneNo,
 WorkPhoneNo,
 MobilePhoneNo,
 CreatedBy,
 DateCreated,
 UpdatedBy,
 DateLastUpdated,
 UpdateId
)

SELECT
 PersonId,
 TitleId,
 FirstName,
 FamilyName ,
 FullName,
 AltFamilyName,
 Sex,
 DateOfBirth,
 Age,
 DateOfDeath,
 CauseOfDeathId,
 Height,
 Weight,
 ABO,
 RhD,
 Comments,
 LocalIdNo,
 NHSNo,
 CHINo,
 HospitalId,
 HospitalNo,
 AltHospitalId,
 AltHospitalNo,
 EthnicGroupId,
 CitizenshipId,
 NHSEntitlement,
 HomePhoneNo,
 WorkPhoneNo,
 MobilePhoneNo,
 CreatedBy,
 DateCreated,
 UpdatedBy,
 DateLastUpdated,
 UpdateId

 FROM vw_GetPersonsByCategoryAndType '

EXEC (@SelectClause + @WhereClause +@OrderByClause)