Unexpected end of string when removing key from hstore - postgresql

Question

When I am removing a key from a HSTORE, I get the error 'Unexpected end of string':
DB=# UPDATE mytable SET properties = properties - 'key' where label = '9912345678';
ERROR: Unexpected end of string
LINE 1: UPDATE mytable SET properties = properties - 'key' where ...
When I explicitly cast that string, it does work:
DB=# UPDATE mytable SET properties = properties - 'key'::text where label = '9912345678';
UPDATE 1
Why does it give this error message? Isn't 'key' a TEXT column? Or at least a string with a expected end?

Answer 1

The operator is overloaded so the right-hand operand should be explicitly cast. Note the example usage for text, text[] and hstore in the documentation:
'a=>1, b=>2, c=>3'::hstore - 'b'::text
'a=>1, b=>2, c=>3'::hstore - ARRAY['a','b']
'a=>1, b=>2, c=>3'::hstore - 'a=>4, b=>2'::hstore
In the statement
UPDATE mytable SET properties = properties - 'key' where label = '9912345678';
the string 'key' may be resolved as text or hstore. The first choice of the hstore algorithm is hstore, so the statement is resolved to
UPDATE mytable SET properties = properties - 'key'::hstore where label = '9912345678';
and raises the error
ERROR: Unexpected end of string
though the error message could be more informative, like Unexpected end of string while parsing hstore constant.