diff --git a/fsck.c b/fsck.c
index 640422a6c6..c004a37372 100644
--- a/fsck.c
+++ b/fsck.c
@@ -84,26 +84,34 @@ static struct {
 };
 #undef MSG_ID
 
+static void prepare_msg_ids(void)
+{
+	int i;
+
+	if (msg_id_info[0].downcased)
+		return;
+
+	/* convert id_string to lower case, without underscores. */
+	for (i = 0; i < FSCK_MSG_MAX; i++) {
+		const char *p = msg_id_info[i].id_string;
+		int len = strlen(p);
+		char *q = xmalloc(len);
+
+		msg_id_info[i].downcased = q;
+		while (*p)
+			if (*p == '_')
+				p++;
+			else
+				*(q)++ = tolower(*(p)++);
+		*q = '\0';
+	}
+}
+
 static int parse_msg_id(const char *text)
 {
 	int i;
 
-	if (!msg_id_info[0].downcased) {
-		/* convert id_string to lower case, without underscores. */
-		for (i = 0; i < FSCK_MSG_MAX; i++) {
-			const char *p = msg_id_info[i].id_string;
-			int len = strlen(p);
-			char *q = xmalloc(len);
-
-			msg_id_info[i].downcased = q;
-			while (*p)
-				if (*p == '_')
-					p++;
-				else
-					*(q)++ = tolower(*(p)++);
-			*q = '\0';
-		}
-	}
+	prepare_msg_ids();
 
 	for (i = 0; i < FSCK_MSG_MAX; i++)
 		if (!strcmp(text, msg_id_info[i].downcased))