Report definition not valid error 2015

After updating our ssrs 2012 environment to visual studio 2015 I sometimes get an error when deploying a report.

Error rsProcessingError : The definition of this report is not valid or supported by this version of Reporting Services. The report definition may have been created with a later version of Reporting Services, or contain content that is not well-formed or not valid based on Reporting Services schemas. Details: The report definition has an invalid target namespace ‘’ which cannot be upgraded.