diff --git a/core/lib/libtomcrypt/src/pk/asn1/der/utf8/der_decode_utf8_string.c b/core/lib/libtomcrypt/src/pk/asn1/der/utf8/der_decode_utf8_string.c
index a97f57b0624..8c42cd4a84f 100644
--- a/core/lib/libtomcrypt/src/pk/asn1/der/utf8/der_decode_utf8_string.c
+++ b/core/lib/libtomcrypt/src/pk/asn1/der/utf8/der_decode_utf8_string.c
@@ -66,7 +66,7 @@ int der_decode_utf8_string(const unsigned char *in,  unsigned long inlen,
       /* count number of bytes */
       for (z = 0; (tmp & 0x80) && (z <= 4); z++, tmp = (tmp << 1) & 0xFF);
 
-      if (z > 4 || (x + (z - 1) > inlen)) {
+      if (z == 1 || z > 4 || (x + (z - 1) > inlen)) {
          return CRYPT_INVALID_PACKET;
       }