There were seven Invalid Corps Regiments in Washington at the time of the attack.
The 10th V.R.C. was sent to Baltimore in anticipation of an attack there, and the 18th V.R.C. was sent to Laurel, Maryland to protect the B&O railroad.
Note: I use the term Invalid Corps and Veteran Reserve Corps interchangeably. However, after 1863 they were only officially referred to by the latter name.