osslsigncode throws error "Unrecognized file type: initrd.img"

I am trying to sign my bzImage and initrd for secure boot. I am able to sign the bzImage using the osslsigncode, but when I sign the initrd.img, I am presented with the following error :

Unrecognized file type: initrd.img
140265667176432:error:0D0680A8:asn1 encoding routines:ASN1_CHECK_TLEN:wrong tag:tasn_dec.c:1220:
140265667176432:error:0D07803A:asn1 encoding routines:ASN1_ITEM_EX_D2I:nested asn1 error:tasn_dec.c:386:Type=PKCS7
140265667176432:error:0906D06C:PEM routines:PEM_read_bio:no start line:pem_lib.c:707:


Why I am getting this failure? Is there a way I can overcome this error?

I will be loading these signed kernel and initrd from GRUB, where GRUB will be requested Rhboot shim to verify them.

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