Error converting datatype varchar to int

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SQL

C#5.0

In my data layer, I have a method NewCaseNumber to create a system-gen number using stored proc. I get error converting datatype varchar to int. My caseType table has SP-ComplexCrimes. But Im only retreiving the SP portion.
This storedProc generates 2015SP00001. I tested it and it executes fine in db

ALTER PROCEDURE [dbo].[GetNewCaseNumber]  
	-- Add the parameters for the stored procedure here
	@DateOpened as datetime,
	@CaseType as int,
	@NewIDOUT varchar(12)	OUTPUT

AS
BEGIN
SET NOCOUNT ON;

	-- Declare the return variable here
	DECLARE @InvType char(2)
	DECLARE @NewID varchar(12)
	DECLARE @NextNum char(5)

	-- Add the T-SQL statements to compute the return value here
	SELECT @InvType = left(Description,2)
		FROM CaseTypeList
		WHERE ID = @CaseType

	SELECT @NextNum = right('00000' + cast(isnull(max(cast(right(i.InvestigationNum,5) as int)),0) + 1 as varchar(5)),5)
		FROM Investigation i inner join CaseTypeList c on i.CaseType = c.ID
		WHERE substring(i.InvestigationNum,5,2) = @InvType and left(i.InvestigationNum,4) = Year(@DateOpened)

	SELECT @NewID = cast(Year(@DateOpened) as char(4)) + @InvType + @NextNum 

	-- Return the result of the function
	SET @NewIDOUT = @NewID;
END

In my DAL, error is from the ExecuteScalar line

public static string GetNewCaseNumber(DateTime DateOpened, Int32 caseType, Int32 NewIDOUT)
	{
		Database db = DatabaseFactory.CreateDatabase();
		DbCommand dbCommand = db.GetStoredProcCommand("GetNewCaseNumber");

		db.AddInParameter(dbCommand, "@DateOpened", DbType.DateTime, DateTime.Now.Date);
		db.AddInParameter(dbCommand, "@CaseType", DbType.Int32, caseType);
		db.AddOutParameter(dbCommand, "@NewIDOUT", DbType.Int32, NewIDOUT);
		
	return (string)db.ExecuteScalar(dbCommand);
	
	}

Modified to below

public static string GetNewCaseNumber(DateTime DateOpened, Int32 caseType, Int32 NewIDOUT)
	{
        int Result;

        Database db = DatabaseFactory.CreateDatabase();
        DbCommand dbCommand = db.GetStoredProcCommand("GetNewCaseNumber");

	  db.AddInParameter(dbCommand, "@DateOpened", DbType.DateTime, DateTime.Now.Date);
	  db.AddInParameter(dbCommand, "@CaseType", DbType.Int32, caseType);
	  db.AddOutParameter(dbCommand, "@NewIDOUT", DbType.Int32, NewIDOUT);

        db.ExecuteNonQuery(dbCommand);

        Result = (Int32)db.GetParameterValue(dbCommand, "@NewIDOUT");

//New error: Not all code paths return a value

I need help doing it properly, co-worker suggested using ExecuteNonQuery, but I need to return it as a string 2015SP00001. Any help is appreciated, or just point me in the right direction. Thank you!