error invoking procedure in another schema containing database link

Scenario

2 oracle 12C databases:

on one database: end-user schema A runs a procedure in schema B with the usual default definers rights; A  has execute privileges on procedure B so A can run procedure in B fine until:

procedure in schema B is changed to access table in another database using private database link owned by schema B

this procedure compiles ok; and runs ok invoked by B; but now when invoked from A it falls over at run time with:

ORA-02019: connection description for remote database not found

this is even though this is DEFINER rights procedure; maybe this is a security feature where database links are required to be accessible by  invoker (??); can't see any reference to this in documentation