What kind of SD card to buy for GPS Unit (TomTom One)?

I recently bought my husband a TomTom One GPS unit. It says to put in an SD card before powering it up. However, the unit didn’t come with an SD card. Does the card have to be a specific kind, say, pre-loaded with certain information? Or will any SD card do?
I tried a blank SD card and it didn’t work. However, the unit seems to be working fine without a card.

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