Reproduction steps:
1. Open the attached project "Material animation bug report.zip";
2. Open the MaterialAnimation.unity scene and enter a play mode;
3. Observe the console.

Expected result: the material of a sprite renderer should be changed using the property "Material Reference[0]".
Actual result: the error appears in the console (instead of changing the material):
"PPtr cast failed when dereferencing! Casting from Material to Sprite!"

Reproduced on: 2017.1.0p3, 2017.2.0b9, 2017.3.0a6.
Not reproduced: 5.5.3p3, 5.6.3p2, 2017.1.0p2.
Regression firstly introduced: 2017.1.0p3.

Fixed in: 2017.1.1p3, 2017.3.0a8.