diff --git a/library/x509_crt.c b/library/x509_crt.c
index ca8b4649e..493d6334f 100644
--- a/library/x509_crt.c
+++ b/library/x509_crt.c
@@ -574,7 +574,8 @@ static int x509_get_crt_ext( unsigned char **p,
         end_ext_data = *p + len;
 
         /* Get extension ID */
-        if( ( ret = mbedtls_asn1_get_tag( p, end, &extn_oid.len, MBEDTLS_ASN1_OID ) ) != 0 )
+        if( ( ret = mbedtls_asn1_get_tag( p, end_ext_data, &extn_oid.len,
+				  MBEDTLS_ASN1_OID ) ) != 0 )
             return( MBEDTLS_ERR_X509_INVALID_EXTENSIONS + ret );
 
 	extn_oid.tag = MBEDTLS_ASN1_OID;