A strict unit test SHOULD have caught your bug here in code coverage:
...final important step is to analyze and maximize code coverage. We recommend that you implement a dynamic coding standard requiring that all code must have 100% line coverage upon check in to source control.

but of course still with 100%CC, bugs may slip through due to the Quis Custodiet Ipsos Custodes (Who watches the watchmen?) problem of writing test software....

The issue of boxing/unboxing is not black/white, there is a spectrum of "grey goo" in between...

/ allan