[R] margin text warning message NAs coercion

[e-letter]

Readers,

I tried to the following commands:

plot(y~x,ylab=expression(A[1]~B[2],xlab=expression(C~D))
mtext(expression(A[1]~B[2]),"additional text",side=3,line=1)

I receive the text that I want, but the command terminal shows the
following response:

Warning message:
NAs introduced by coercion in: mtext(expression(A[1]~B[2]),"additional
text", side = 3,

What is my mistake please?

Yours,

r251
rhelpatconference.jabber.org