diff --git a/new/doc.jam b/new/doc.jam
index 5f6d06edf..abf01f421 100644
--- a/new/doc.jam
+++ b/new/doc.jam
@@ -598,10 +598,12 @@ local rule scan-variable (
         if $(var-parts[2]) = "?="
         {
             default-value = $(value) ;
+			default-value ?= "(empty)" ;
         }
         else
         {
             initial-value = $(value) ;
+			initial-value ?= "(empty)" ;
         }
         if $(comment-block)
         {
diff --git a/v2/util/doc.jam b/v2/util/doc.jam
index 5f6d06edf..abf01f421 100644
--- a/v2/util/doc.jam
+++ b/v2/util/doc.jam
@@ -598,10 +598,12 @@ local rule scan-variable (
         if $(var-parts[2]) = "?="
         {
             default-value = $(value) ;
+			default-value ?= "(empty)" ;
         }
         else
         {
             initial-value = $(value) ;
+			initial-value ?= "(empty)" ;
         }
         if $(comment-block)
         {