diff --git a/template/exts.mk.tmpl b/template/exts.mk.tmpl
index 1102219c00..a9a9ea09d1 100644
--- a/template/exts.mk.tmpl
+++ b/template/exts.mk.tmpl
@@ -75,7 +75,7 @@ if gnumake
   submake = "$(MAKE) -C $(@D)"
 else
   submake = "cd $(@D) && "
-  exec = RbConfig::CONFIG["exec"] and !exec.empty? and submake << str << " "
+  exec = RbConfig::CONFIG["exec"] and !exec.empty? and submake << exec << " "
   submake << "$(MAKE)"
 end
 %>