diff --git a/config.c b/config.c
index 5eca17b24..788970c6b 100644
--- a/config.c
+++ b/config.c
@@ -63,8 +63,12 @@ static int config_buf_fgetc(struct config_source *conf)
 
 static int config_buf_ungetc(int c, struct config_source *conf)
 {
-	if (conf->u.buf.pos > 0)
-		return conf->u.buf.buf[--conf->u.buf.pos];
+	if (conf->u.buf.pos > 0) {
+		conf->u.buf.pos--;
+		if (conf->u.buf.buf[conf->u.buf.pos] != c)
+			die("BUG: config_buf can only ungetc the same character");
+		return c;
+	}
 
 	return EOF;
 }