Phalcon seemingly converting NULL values to empty strings..?

I am unsure as to whether I have missed a significant under the hood change, or if this is a bug.

I just revisited a project that I had not run in a long time. Running my test suite resulted in a number of failures when testing that an exception is thrown upon trying to insert a NULL value into a NOT NULL MYSQL column.

It seems as though NULL values are being converted to empty strings ('') and then inserted successfully rather than throwing the expected exception..

Can anyone share any insights as to why this is?

Thanks T