Adding 2 columns with '+' treats numbers as strings (not fixed with cast)

Adding 2 columns with '+' treats numbers as strings (not fixed with cast)

Why does the '+' sign converts numbers into strings? How to fix it?
Note: cast( as int) + cast( as int) does not work ("Parsing of Query failed, please check the syntax").

The following SQL should sum up two numbers, like: 0 + 10000
Expected result: 10000
Actual result: "010000"

  3. case when Type='Renewal' and sum("No. Licenses") over(partition by "Account Name" ORDER BY "Contract Start Date") > 0 then "No. Licenses"
  4. else sum(case when Type!='Renewal' then "No. Licenses" else 0 end) 
  5. over(partition by "Account Name"  ORDER BY "Contract Start Date" rows between unbounded precedent and current row )
  6. end 
  8. + 
  10. case when Type!='Renewal' and "Contract Start Date" != first_value("Contract Start Date") over(partition bypartition by "Account Name")
  11. and first_value("No. Licenses") over(partition by "Account Name"  ) = 0 
  12. then first_value("No. Licenses") over(partition by "Account Name"  ORDER BY case when "No. Licenses"  = 0 then 2 else 1 end, "Contract Start Date" )
  13. else 0
  14. end as cummulative_licenses