i2c device working on Arduino Mega but not Arduino Micro

I'm using an RFID/NFC device that using I2C in an arduino project. I was using the adafruit shield, but it's too large for what I'm trying to do. I found this PN532 NFC RFID module kits -- Arduino compatible [WIRELESS-NFC-PN532] - $22.90 : Elechouse, Arduino Play House . It's pretty straighforward to connect and has a decent library that I was able to get working with an arduino mega. When I tried moving it to an arduino micro, I got no repsonse from the device at all. I was able to get the adafruit RFID shield working with a micro using i2c, so it's not just me using the wrong pins or something.

Does the micro handle i2c differently somehow? I'm crawling through the library for this new RFID board right now, and I can't see anything that's board specific. Thanks!

pullups installed ?

I wasn't aware that the micro didn't have pullups installed. I just scoped the SDA line and it's a bunch of triangle waves, so that's definitely the problem. What's a good value pullup resistor? Thanks!

3k3.. 10k