"-M" option does not work with "+" "-" characters

[Hyunho Cho]

### without "-M" option work well as expected

bash$ awk '{ if ($1) print "must print " $1 }'       
1                  <------ user input
must print 1       
2                  <------ user input
must print 2
+
must print +       # work as expected
-
must print -       # work as expected
^C


### with "-M" option "+" "-" characters do not print

bash$ awk -M '{ if ($1) print "must print " $1 }' 
1
must print 1
2
must print 2
+                  # does not print
-                  # does not print
*
must print *
/
must print /
^C