error adding 0.1



If I try to do:
(%i10) 1000!+1;
it works, while if I try this, I get an error message, 

(%i10) 1000!+0.1;

Maxima encountered a Lisp error:

 Error in PROGN [or a callee]: Can't print a non-number.

Automatically continuing.
To enable the Lisp debugger set *debugger-hook* to nil.

why?