Ok. My issue is resolved.
I created a new deployment share in MDT and then configured it. In doing so, MDT 2013 required that I install Windows ADK for Windows 10. (I previously had Windows ADK for Windows 8.1 installed.) After these steps, I tried again to create a boot image using MDT, and did not receive the error.
Also, if you are using ADK for Windows 10 to generate boot images for SCCM using MDT, you may encounter this issue too: https://support.microsoft.com/en-us/kb/3143760