It seems like $target defined by parameter $target on line 51 can also be of type object; however, JMS\Serializer\Serializer::fromArray() does only seem to accept string, maybe add an additional type check?
This check looks at variables that have been passed in as parameters and are passed out again
to other methods.
If the outgoing method call has stricter type requirements than the method itself, an issue is raised.
It seems like the type of the argument is not accepted by the function/method which you are calling.
In some cases, in particular if PHP’s automatic type-juggling kicks in this might be fine. In other cases, however this might be a bug.
We suggest to add an explicit type cast like in the following example: