diff --git a/Makefile.am b/Makefile.am
index b3f3366e79eb1c402ec70f3fdfea971e907d77f9..119116b9d1dd5e3ebb800e438a97b164aabc4dc4 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -123,7 +123,7 @@ generate-alberta-dist:
 #
 # The documentation is available as book.
 #
-EXTRA_DIST = doc/alberta.pdf
+EXTRA_DIST += doc/alberta.pdf
 
 #
 # this one requires write access to the source directory. One day one might