services:
    _defaults:

If you leave _defaults empty (i.e. null), you got a bad error before. Now it's better :)

Before:

The definition for "_defaults" has no class. If you intend to inject this service dynamicall
y at runtime, please mark it as synthetic=true. If this is an abstract definition solely use
d by child definitions, please add abstract=true, otherwise specify a class to get rid of th
is error.

After:

Service "_defaults" key must be an array, "NULL" given in "/path/to/services.yml"