puppet upgrade problem 0.25 "could not convert from pson"

After upgrading to 0.25, the following error occurs:

Could not retrieve catalog from remote server: Could not intern from pson: Could not convert from pson: Could not find relationship target ''

This turned out to be because of recipes using exec without naming the exec. Example

exec { "cat /that/file": unless => "something", path => "/bin", refreshonly => false }

should be rewritten as

exec { "cat that file": command => "cat /that/file", unless => "something", path => "/bin", refreshonly => false }

The error goes away after making the change and all is well.