The effect of Inzektor Exa-Stag says:

"Once per turn: You can detach 1 Xyz Material from this card to target 1 monster your opponent controls or in their Graveyard; equip that target to this card. This card gains ATK and DEF equal to half the original ATK and DEF of the monster(s) equipped by this effect."

But in the game, in addition to monsters, he is able to select spell cards in the grave as a target of his effect, the spell card is not equipped after that and the effect resolves without anything happening in the end.

I've not tested if this happens with traps too.

Here some images:

I can activate it even if the opponent do not have a monster in the field or graveyard

Spellbook can be equipped