Function is_hash not defined despite being loaded!

I get this error on the server:

Error: Could not retrieve catalog from remote server: Error 400 on SERVER: Evaluation Error: Error while evaluating a Function Call, Function is_hash not defined despite being loaded! at /etc/puppet/environments/goosy/modules/apache/manifests/vhost.pp:410:9

(puppetforge module versions):
stdlib 4.2.0
apache 1.0.1

(puppet versions)
client: 3.6.1
server: 3.6.1
facter 2.0.1
hiera 1.3.3

It worked fine before the upgrade to 3.6 series. AND I don't get the error when I do a Puppet apply just on the client.