MSSQL says I am trying to convert a varchar to a string when I'm not - sql

Question

So I have this fairly long procedure at Work that I just made. What it does it not that important, but the end result is what matters.
I need to count some different types of descriptions in a table and that Works fine. I then need to take the two things that I Count and put them in a string that I return to my software. However, every time I run this procedure it gives me this:
Msg 245, Level 16, State 1, Procedure WorkDays, Line 43 Conversion
failed when converting the varchar value
'FlightDeck:161,CabinCrew:189' to data type int.
I just can't figure out why it keeps telling me this when I am not trying to convert a varchar to an int but rather ints to a single varchar.
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
ALTER PROCEDURE [dbo].[WorkDays] #requestedDate nchar(10)
AS
SET ANSI_WARNINGS OFF
DECLARE #date as nchar(10) = ''
DECLARE #returnVal as varchar(30) = ''
DECLARE #flightDeck as int = 0
DECLARE #cabinCrew as int = 0
BEGIN
-- SET NOCOUNT ON added to prevent extra result sets from
-- interfering with SELECT statements.
SET NOCOUNT ON;
SET #date = #requestedDate
SELECT
#flightDeck = SUM(CASE WHEN dbo.Crew_Category.Description LIKE 'Flight Deck' THEN 1 END),
#cabinCrew = SUM(CASE WHEN dbo.Crew_Category.Description LIKE 'Cabin Crew' THEN 1 END)
FROM
dbo.CrewMember INNER JOIN
dbo.Crew_Category ON dbo.CrewMember.CrewCategorySeqNo = dbo.Crew_Category.CrewCategorySeqno
WHERE
(dbo.Crew_Category.Description = N'Flight Deck' OR
dbo.Crew_Category.Description = N'Cabin Crew') AND
(dbo.CrewMember.EmploymentEndDate > #date)
AND dbo.CrewMember.CrewSeqno NOT IN (
SELECT
CrewMember_1.CrewSeqno
FROM
dbo.CrewMember AS CrewMember_1 INNER JOIN
dbo.CrewReqAsg ON CrewMember_1.CrewSeqno = dbo.CrewReqAsg.crewSeqno INNER JOIN
dbo.activity ON dbo.CrewReqAsg.act_seqno = dbo.activity.act_seqno INNER JOIN
dbo.ActivityType ON dbo.activity.actType_seqno = dbo.ActivityType.actType_seqno INNER JOIN
dbo.ActivityCategory ON dbo.ActivityType.ActCat_seqno = dbo.ActivityCategory.actCat_seqno INNER JOIN
dbo.Crew_Category AS Crew_Category_1 ON CrewMember_1.CrewCategorySeqNo = Crew_Category_1.CrewCategorySeqno
WHERE (
dbo.ActivityCategory.Category = N'Ferie' OR
dbo.ActivityCategory.Category = N'Fridage' OR
dbo.ActivityCategory.Category = N'Sygdom') AND (Crew_Category_1.Description = N'Flight Deck' OR
Crew_Category_1.Description = N'Cabin Crew') AND (LEFT(dbo.activity.Start,10) LIKE #date));
SET #returnVal = 'FlightDeck:'+CAST(#flightDeck AS varchar);
SET #returnVal += ',CabinCrew:'+CAST(#cabinCrew AS varchar);
END
RETURN #returnVal
It's been a while since I've had to do this so perhaps I just forgot something fundamental. Please help me figure out why this happens? :)

Answer 1

Yes, you forgot something fundamental. To return data to the caller, use SELECT, not RETURN.
You need
SELECT #returnVal