Proxy signature is a variant of digital signature, in which the proxy signer could sign message on behalf of the original signer. In 2007, Hu et al. introduced a mechanism into the proxy signature model to allow rebuilding the short-term proxy key in a very efficient way. The mechanism reflects the necessary while the content stated in the warrant has been changed. Unfortunately, the new mechanism solves the demand on changing warrant but introduce a new security issue that the genuine long-term secret key of the proxy signer will be exposed if one of the proxy keys is compromised. Therefore, the impersonation attack is possible in Hu et al.'s scheme. In this paper, the security weakness will be examined and remedied. The enhanced version still retains the merits of the original design and this Elliptic curve based scheme is definitely adaptive for the mobile environment to provide a security solution for the next century.