diff --git a/tool/rbinstall.rb b/tool/rbinstall.rb
index 399ca67..22ead9b 100755
--- a/tool/rbinstall.rb
+++ b/tool/rbinstall.rb
@@ -360,8 +360,8 @@ install?(:ext, :arch, :'ext-arch') do
   prepare "extension objects", archlibdir
   noinst = %w[-* -*/] | (CONFIG["no_install_files"] || "").split
   install_recursive("#{$extout}/#{CONFIG['arch']}", archlibdir, :no_install => noinst, :mode => $prog_mode, :strip => $strip)
-  prepare "extension objects", sitearchlibdir
-  prepare "extension objects", vendorarchlibdir
+  prepare "extension objects", sitearchlibdir if sitearchlibdir
+  prepare "extension objects", vendorarchlibdir if vendorarchlibdir
 end
 install?(:ext, :arch, :'ext-arch') do
   prepare "extension headers", archhdrdir
@@ -370,8 +370,8 @@ end
 install?(:ext, :comm, :'ext-comm') do
   prepare "extension scripts", rubylibdir
   install_recursive("#{$extout}/common", rubylibdir, :mode => $data_mode)
-  prepare "extension scripts", sitelibdir
-  prepare "extension scripts", vendorlibdir
+  prepare "extension scripts", sitelibdir if sitelibdir
+  prepare "extension scripts", vendorlibdir if vendorlibdir
 end
 install?(:ext, :comm, :'ext-comm') do
   hdrdir = rubyhdrdir + "/ruby"