diff --git a/configure.ac b/configure.ac
index 6df07067a..35a7dfa47 100644
--- a/configure.ac
+++ b/configure.ac
@@ -127,6 +127,10 @@ AC_SUBST(LT_NGSPICE_REVISION)
 AC_SUBST(LT_NGSPICE_AGE)
 AC_SUBST(LIBNGSPICE_SO_VERSION)
 
+# Defining the API Version
+LIBNGSPICE_API_VERSION=0
+AC_SUBST(LIBNGSPICE_API_VERSION)
+
 # Package Options
 # ---------------