Using property values in QML animations triggers an assertion

I was trying to make a button wiggle back and forth to grab my users attention. I was triggering the animation using a property (wiggle_value in this case). If I use the wiggle_value property instead of an integer in the second `NumberAnimation` during my `SequentialAnimation`, it messes with the animation. The first time everything animates as expected. The second and subsequent animations are visually different. Stunted, almost. It only doing half the rotation, and the timing is weird.

In the attached code, if you change line 45 of `main.qml` from

NumberAnimation {
    to: -ready_button.wiggle_value
    duration: 120
}

to the below

NumberAnimation {
    to: -10
    duration: 120
}

Every works as expected. 

The zip file has all the code zipped together. I attached the main.qml for ease of view as well.