BUG #11553: Sum function does not exist in Create View

The following bug has been logged on the website:

Bug reference:      11553
Logged by:          Harry Clarke
Email address:      Harry@HarryClarke.me.uk
PostgreSQL version: 9.3.5
Operating system:   Windows 8.1 64bit
Description:

When attempting to create a view using the following commands
CREATE VIEW myschema.vc17_tetrad_months AS
 SELECT mv.tetrad,
   sum(mv.month) AS months_visited,
   mv.the_geom
 FROM ( SELECT
  ...
  ) mv
GROUP BY mv.tetrad, mv.the_geom;;

The error message
ERROR:  function sum(text) does not exist
LINE 5:     sum(mv.month) AS months_visited,
            ^
HINT:  No function matches the given name and argument types. You might need
to add explicit type casts.

is returned. However, if the function "sum" is replaced by the function
"count". No error message is returned, and the expected result is obtained.