diff --git a/src/common/saslprep.c b/src/common/saslprep.c
index e7e909a0c87..80b9edb3968 100644
--- a/src/common/saslprep.c
+++ b/src/common/saslprep.c
@@ -1009,15 +1009,17 @@ pg_utf8_string_len(const char *source)
 	const unsigned char *p = (const unsigned char *) source;
 	int			l;
 	int			num_chars = 0;
+	size_t		len = strlen(source);
 
-	while (*p)
+	while (len)
 	{
 		l = pg_utf_mblen(p);
 
-		if (!pg_utf8_islegal(p, l))
+		if (len < l || !pg_utf8_islegal(p, l))
 			return -1;
 
 		p += l;
+		len -= l;
 		num_chars++;
 	}