plpgsql - cache lookup error 18977

I entered the following to load plpgsql:
"$createlang -L/usr/lib/pgsql plpgsql testdb"
postgresql appeared to have no problem with it. A
subsequent "$createlang -l -dtestdb" showed plpgsql as
being a trusted procedural language.  But whenever I
attempt to use it in a function I get the following
error:
"ERROR: fmgr_info: Cache lookup for language failed
18977"
What does this mean, or where can I find out what this
means?

Thanks.

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