Infra red stretching frequency of carbon monoxide ligand in borane carbonyl compared to that of free carbon monoxide

Why is the infrared stretching frequency of $ce{CO}$ in the borane carbonyl $ce{BH3CO}$ more than that of free $ce{CO}$?

Is there any real explanation for this?

There will be a small amount of hyperconjugation of $ce{H}$ atoms from boron to carbon which would increase the electron density of carbon closer to boron due to which the $ce{CO}$ bond strength should weaken.

Am I wrong anywhere or there is any other theory that explains this?