diff --git a/ext/extmk.rb b/ext/extmk.rb
index 5e86078d7b..0a61b65e84 100644
--- a/ext/extmk.rb
+++ b/ext/extmk.rb
@@ -105,7 +105,7 @@ getopts('', 'extstatic', 'make:', 'make-flags:')
 $force_static = $OPT['extstatic'] == 'static'
 $make = $OPT['make'] || $make
 $mflags = Shellwords.shellwords($OPT['make-flags'] || "")
-$mflags[0].sub!(/^(?=\w+)$/, "-") unless mflags.empty?
+$mflags[0].sub!(/^(?=\w+)$/, "-") unless $mflags.empty?
 $make, *$mflags[0, 0] = Shellwords.shellwords($make)
 
 mflags = $mflags.grep(/^-([^-].*)/) {$1}.join