Error "In a grant or revoke statement, the user attempted to grant a tablespace quota above the upper limit." when trying to increase user quota I'm attempting to increase the space quota on a particular schema. I'm using the following query to do so:

ALTER USER MY_SCHEMA QUOTA "100 M" ON "USERS";


This results in the following error:

SQL Error: ORA-02187: invalid quota specification
02187. 00000 -  "invalid quota specification"
*Cause:    In a grant or revoke statement, the user attempted to grant
           a tablespace quota above the upper limit.
*Action:   Grant a smaller tablespace quota.


But this doesn't make any sense. The user already has a 50 MB quota, and a query trying to set it to 25 MB fails with the same error message:

ALTER USER MY_SCHEMA QUOTA "25 M" ON "USERS";


I've confirmed that my tablespace has enough room left. (Its data files can acquire about 2 GB more of disk space.) What is causing this error?
It turns out that the error message is misleading. The problem is the quoting around the size specification.

Both of these work:

ALTER USER MY_SCHEMA QUOTA 100M ON "USERS";


and 

ALTER USER MY_SCHEMA QUOTA 100 M ON "USERS";