I think it should be invalid? The guarantee is performed unilaterally, and neither the lender nor the borrower has fulfilled their obligations to the guarantor, so the real society has nothing to do with the guarantor. Therefore, there is no possibility of guarantee relationship in real society. Therefore, the guarantor should clearly indicate the voluntary guarantee and sign it on the iou to ensure the establishment of the guarantee relationship. If the contents indicated on the IOU are vague, the guarantee relationship is unclear and the identity information is unclear, it is difficult to determine the guarantee. There is no clear indication that someone is willing to guarantee. The guarantee relationship is uncertain and the guarantee is invalid. After all, there is no guarantee obligation relationship in the real society, and there is no benefit from it. The signature is vague, which does not clearly indicate that someone voluntarily guarantees and establishes a guarantee relationship. Such a signature is invalid.