diff --git a/configure.ac b/configure.ac
index d11aae2170..4e118b7813 100644
--- a/configure.ac
+++ b/configure.ac
@@ -64,6 +64,7 @@ AC_ARG_WITH(baseruby,
 	[
 		AC_PATH_PROG([BASERUBY], [ruby], [false])
 	])
+# BASERUBY must be >= 2.2.0. Note that `"2.2.0" > "2.2"` is true.
 AS_IF([test "$HAVE_BASERUBY" != no -a "`RUBYOPT=- $BASERUBY --disable=gems -e 'print 42 if RUBY_VERSION > "2.2"' 2>/dev/null`" = 42], [
     AS_CASE(["$build_os"], [mingw*], [
         # Can MSys shell run a command with a drive letter?