diff --git a/boostbook.jam b/boostbook.jam
index 9e3645bf0..fa202d0bb 100644
--- a/boostbook.jam
+++ b/boostbook.jam
@@ -67,8 +67,14 @@ rule init ( docbook-xsl-dir ? : docbook-dtd-dir ? )
   if ! $(.initialized) 
   {
     $(.initialized) = true ;
-    .docbook-xsl-dir = [ path.make $(docbook-xsl-dir) ] ;
-    .docbook-dtd-dir = [ path.make $(docbook-dtd-dir) ] ;
+    if $(docbook-xsl-dir) 
+    {
+      .docbook-xsl-dir = [ path.make $(docbook-xsl-dir) ] ;
+    }
+    if $(docbook-dtd-dir) 
+    {
+      .docbook-dtd-dir = [ path.make $(docbook-dtd-dir) ] ;
+    }
     local boost-root = [ path.make [ modules.peek : BOOST_ROOT ] ] ;
     .boostbook-xsl-dir = [ path.join $(boost-root) tools boostbook xsl/ ] ;
     .boostbook-dtd-dir = [ path.join $(boost-root) tools boostbook dtd/ ] ;