diff --git a/configure.ac b/configure.ac
index 0835c7406..210dc8e89 100644
--- a/configure.ac
+++ b/configure.ac
@@ -12,7 +12,7 @@
 # ---------------
 
 # Initialization of configure
-AC_INIT([ngspice], [33], [http://ngspice.sourceforge.net/bugrep.html])
+AC_INIT([ngspice], [33+], [http://ngspice.sourceforge.net/bugrep.html])
 
 # Unique file in the source directory
 AC_CONFIG_SRCDIR([src/ngspice.c])