Japanese novelist in the mid 20th century theorized that Uesugi Kenshin was a woman after he discovered a report on 16th century Japan in the monastery of Toledo which was used as a fort during the Spanish Civil War. This report about Japan was written by a person named Gonzalez of Spain to King Philip II. In that report, Gonzalez refers to certain Uesugi as "tia" (aunt) of Uesugi Kagekatsu, the biological son of Kenshin's sister. The discovery of this letter made the novelist theoritise that "in the West, Uesugi Kenshin is a woman". On this basis, Yagiri wrote the "" (Female Uesugi Kenshin theory). Other evidence for this theory is that Kenshin had severe stomach cramps on a monthly basis around the 10th of the month (recorded in the Kōyō Gunkan) and actually planned his military campaigns around these cramps. The cause of Kenshin's death is recorded in Matsudaira Tadaaki's history "" (当代記) as a form of uterine cancer (大虫).