Mule AMQP connector: Value is not facet-valid with respect to enumeration -

i'm trying read property configuration file (.properties) in mule seems not take value it.

these configuration , properties files:

dev.properties file

amqp.host=localhost amqp.port=5672 amqp.username=guest amqp.password=guest amqp.ackmode=manual amqp.prefetchcount=1 

xml configuration file

<context:property-placeholder location="dev.properties"/>  <amqp:connector      name="amqplocalhostconnector"     host="${amqp.host}"     port="${amqp.port}"     username="${amqp.username}"     password="${amqp.password}"     ackmode="${amqp.ackmode}"     prefetchcount="${amqp.prefetchcount}" /> 

when execute application, throws following error:

error 2015-06-29 12:04:33,540 [main] org.mule.module.launcher.application.defaultmuleapplication: null org.xml.sax.saxparseexception: cvc-enumeration-valid: value '${amqp.ackmode}' not facet-valid respect enumeration '[amqp_auto, mule_auto, manual]'. must value enumeration. 

if set property this:

    ackmode="manual" 

the application runs without problem.

how can work? thanks.