diff --git a/tool/update-deps b/tool/update-deps
index 7c4d5d3319..f26cff26d5 100755
--- a/tool/update-deps
+++ b/tool/update-deps
@@ -239,7 +239,7 @@ DEPENDENCIES_SECTION_END_MARK = "\# AUTOGENERATED DEPENDENCIES END\n"
 
 def init_global
   ENV['LC_ALL'] = 'C'
-  if mkflag = ENV['GNUMAKEFLAGS'] and mkflag.sub!(/(\A|\s+)-j\d*(?=\s+|\z)/, '')
+  if mkflag0 = ENV['GNUMAKEFLAGS'] and (mkflag = mkflag0.sub(/(\A|\s+)-j\d*(?=\s+|\z)/, '')) != mkflag0
     mkflag.strip!
     ENV['GNUMAKEFLAGS'] = mkflag
   end