From Modern Hebrew Grammar: The infinitive
a Form of the infinitive
The infinitive cannot be inflected. Whether one is addressing males or
females, one person or many, it is unchanged.
The infinitive’s distinguishing mark is a prefixed lamed ל ...
In other instances, it said "mares" when they wanted us to say סוסות. I believe that is why it is not accepted here, because it says simply "horses" and masculine is the default gender of most animals.