Model with no primary key
Bug confirmed, curiously I just came across this issue myself and was about to report it, weird coincidence? Or perhaps a recent change?

Also, specifying `primary_key = None` doesn't actually disable the PK, as seen here;

``` py
class Audit(Model):
    class Meta:
        primary_key = False
    data = TextField()

>>> a = Audit(data="lol")
>>> a.save()
>>> a.id
1
```
 Setting `primary_key = None` will cause peewee to auto-generate an `id` primary key field, so the docs are correct regarding `False`.

Unfortunately some APIs just don't work when you don't have any primary key at all, `save()` being one of them. You need to use the `.insert()` and `.update()` APIs instead.
 Added some docs to that effect. 
 Ah yeah, makes sense now, thanks for the quick reply!

It looks like while this is supported the documentation might be incorrect, I believe what the documentation actual intends is:

``` python
class MyData(BaseModel):
    timestamp = DateTimeField()
    value = IntegerField()

    class Meta:
        primary_key = None
```

While:

``` python
    primary_key = False
```

Seems to cause issues when trying to issue a .save() command.
