diff --git a/build-aux/configure-lib.sh b/build-aux/configure-lib.sh
index c28b339c..e4b8943d 100644
--- a/build-aux/configure-lib.sh
+++ b/build-aux/configure-lib.sh
@@ -113,6 +113,7 @@ if test $mes_libc = mes; then
 lib/ctype/isdigit.c
 lib/ctype/isspace.c
 lib/ctype/isxdigit.c
+lib/mes/assert_msg.c
 lib/posix/write.c
 lib/stdlib/atoi.c
 "
diff --git a/include/mes/mes.h b/include/mes/mes.h
index 422be601..ea49256a 100644
--- a/include/mes/mes.h
+++ b/include/mes/mes.h
@@ -105,6 +105,7 @@ int unreadchar ();
 long length__ (SCM x);
 size_t bytes_cells (size_t length);
 void assert_max_string (size_t i, char const *msg, char *string);
+void assert_msg (int check, char *msg);
 
 #include "mes/builtins.h"
 #include "mes/constants.h"
diff --git a/lib/mes/assert_msg.c b/lib/mes/assert_msg.c
new file mode 100644
index 00000000..191aa87c
--- /dev/null
+++ b/lib/mes/assert_msg.c
@@ -0,0 +1,30 @@
+/* -*-comment-start: "//";comment-end:""-*-
+ * GNU Mes --- Maxwell Equations of Software
+ * Copyright © 2019 Jeremiah Orians <jeremiah@pdp10.guru>
+ * Copyright © 2019 Jan (janneke) Nieuwenhuizen <janneke@gnu.org>
+ *
+ * This file is part of GNU Mes.
+ *
+ * GNU Mes is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or (at
+ * your option) any later version.
+ *
+ * GNU Mes is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with GNU Mes.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <mes/lib.h>
+#include <stdlib.h>
+
+void
+assert_msg (int bool, char *msg)
+{
+  if (bool == 0)
+    __assert_fail (msg);
+}