ST_BOUNDARY in Oracle Spatial

Question

ST_Boundary is in the Oracle Spatial docs here:

https://docs.oracle.com/en/database/oracle/oracle-database/21/spatl/sql-multimedia-type-support.html#GUID-22FE0E11-C9F0-4E1A-B512-D494E588FFAA

But only as an example:

SELECT c.shape.ST_BOUNDARY() 
  FROM cola_markets c WHERE c.name = 'cola_b';

It runs but always returns NULL, is this an oversight or something else? It doesn't throw a syntax issue so something is going on?

SELECT c.shape FROM cola_markets c

Returns a regular ST_GEOMETRY with no issue.