Up until now I had been signing my apks with the following method:
jarsigner -verbose -sigalg SHA1withRSA -digestalg SHA1 -keystore {keystore-file} {apk-file} {key
Here's what worked for me, I changed this:
--ks-pass "MyPassword"
To this:
--ks-pass pass:"MyPassword"
A weird API to work with... but it worked!