SQL Server 2012 :: Scalar Function Returning Zero When It Shouldn't

I have this code:

Declare @sql as varchar(4000)
declare @tbl as varchar(100)
declare @exists as bit
select @tbl = 'ACA_RSF'
select @sql = 'select count(*) from [member_score] where source_tbl = ''' + @tbl + ''''
print @sql
exec (@sql)

and it returns 18 million for a record count.I have this scalar returning function, which models the above, and it returns zero:

select dbo.fnGet_Rec_Count('ACA_RSF') as cnt

here is the code:

alter FUNCTION spGet_Rec_Count
(
@source_tbl varchar(100)
)
RETURNS bigint
AS
BEGIN

-- Declare the return variable here

DECLARE @count bigint

-- Add the T-SQL statements to compute the return value here

select @count = (select count(*) from [member_score] where source_tbl = ''' + @tbl + ''')

-- Return the result of the function

RETURN @count
END
GO

I get zero regardless of where @count is declared as in or bigint.