Magento

Magento 1.7.0.0-beta1, When to enable memcached, get very strange error:

memcached config:

<cache>
<backend>memcached</backend>
<memcached>
<servers>
<server>
<host>127.0.0.1</host>
<port>11211</port>
<persistent>0</persistent>
</server>
</servers>
</memcached>
</cache>

any help?

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