[plt-scheme] "name is not defined, ..." error in non-teaching language?

I received a support query from someone having difficulty with the  
graphics library I supply for using DrScheme with the Concrete  
Abstractions textbook.  I'm confused what's going on and hope someone  
else might be able to figure it out.

The problem is he is getting an error message "name is not defined,  
not a parameter, and not a primitive name" regarding a name that  
should be defined by my library, which he is requiring.  But the  
really weird part -- the reason I'm confused -- is that the error  
message looks like the one that would come up in a teaching language.   
So far as I can tell, in a non-teaching language, the message ought to  
be "reference to undefined identifier", even if the library somehow  
were not supplying the name.  Yet the user in question sent me a  
screen snapshot, in which the language is clearly set to "Textual  
(MzScheme, includes R5RS)".  Moreover, I can see he is running a  
current version of DrScheme and that the error message is as described  
above.  So.... can someone help me out?  Under what circumstances  
would one get an error message "name is not defined, not a parameter,  
and not a primitive name" rather than "reference to undefined  
identifier"?

For reference, I have put the screen snapshot on my web page at http://gustavus.edu/+max/drscheme-error.jpg

Thanks. -max