ANS(fun_cmp("sqrt(1+4x^2) dx", vars=>['x','dx']));results in the warning "Use of uninitialized value in concatenation (.) or string at line 733 of (eval 983)", but seems to work properly. The problem appears to be the dx.

1) Is there really a problem? If so, can I fix it? If not, can I eliminate the warning message (so I won't have all my students asking me about it?)