diff --git a/configure.ac b/configure.ac
index 4f05aeb4..c1c0d5a1 100644
--- a/configure.ac
+++ b/configure.ac
@@ -148,6 +148,8 @@ AC_ARG_WITH(cflags-scanner-extra, [AS_HELP_STRING([--with-cflags-scanner-extra@<
 AC_MSG_NOTICE([extra cflags for scanner.c: $CFLAGS_SCANNER_EXTRA])
 AC_SUBST(CFLAGS_SCANNER_EXTRA)
 
-AC_SUBST(LIBPOSTAL_SO_VERSION, LIBPOSTAL_MAJOR_VERSION:LIBPOSTAL_MINOR_VERSION:LIBPOSTAL_PATCH_VERSION)
+
+LIBPOSTAL_SO_VERSION="${LIBPOSTAL_MAJOR_VERSION}:${LIBPOSTAL_MINOR_VERSION}:${LIBPOSTAL_PATCH_VERSION}"
+AC_SUBST(LIBPOSTAL_SO_VERSION)
 
 AC_OUTPUT