ImportError: No module named yaml

Hello,
I tried the latest version of the server this morning and I have this error :

Traceback (most recent call last):
  File "./openerp/5.2/openobject-server/bin/openerp-server.py", line 54, in <module>
    import netsvc
  File "/home/julien/openerp/5.2/openobject-server/bin/netsvc.py", line 273, in <module>
    import tools
  File "/home/julien/openerp/5.2/openobject-server/bin/tools/__init__.py", line 26, in <module>
    from convert import *
  File "/home/julien/openerp/5.2/openobject-server/bin/tools/convert.py", line 40, in <module>
    from yaml_import import convert_yaml_import
  File "/home/julien/openerp/5.2/openobject-server/bin/tools/yaml_import.py", line 10, in <module>
    import yaml_tag
  File "/home/julien/openerp/5.2/openobject-server/bin/tools/yaml_tag.py", line 1, in <module>
    import yaml
ImportError: No module named yaml

Is it normal ?