in file /src/phpDocumentor/Reflection/DocBlock/Tag/SourceTag.php there's a reference to DocBlock $docblock = null in class constructor, but it seems it's getting it from wrong domain resulting failing instance check.
DocBlock $docblock = null
will solve the problem, but i'm not sure it's the correct way to fix this, because the reason for the wrong reference might be in different place, i got the error both on master and develop branches.
This is a known issue that should already be fixed in the upcoming PhpDocumentor release.
Thank you for the report anyway.
On June 1st we will release the first beta that will include this fix; because no further action is required will I close this issue