bug report: Error: (MPLUS) is invalid as a function.



Anyone can reproduce this? And what the hell does the error message 
means!!!! What is MPLUS!!!

(C5) f(x):=abs(1+x);

(D5)                          f(x) := ABS(1 + x)
(C6) f(1);


Error: (MPLUS) is invalid as a function.
Fast links are on: do (si::use-fast-links nil) for debugging
Error signalled by MACSYMA-TOP-LEVEL.
Broken at LIST.  Type :H for Help.
MAXIMA>>:q