xtmixed error: missing parameter values not allowed

#1
Stata gives me the above error in response to
Code:
. xtmixed IR7 f3 || ID:, mle
[COLOR="red"]missing parameter values not allowed
__00000S not found
[/COLOR]
. list if IR7==. | f3 == . | ID==.

.
There are evidently no missing values in the data.
What parameter is it talking about? I cannot find any documentation about this error.

TFYH