Add faq question #646

docs/faq/why-i-get-a-typeerror-requires-a-single-type.rst

@@ -0,0 +1,28 @@
+Why I get a TypeError: requires a single type
+=============================================
+
+The full error message looks something like this:
+
+.. code-block::
+
+    TypeError: typing.Optional requires a single type. Got Field(name=None,type=None,default=<dataclasses._MISSING_TYPE object at 0x7f79f4b0d700>,default_facto.
+
+The error means the dataclass wrapper can't build the typing annotations for a model
+because the field type is ambiguous. If you are using the code generator make sure you
+are not using the same convention for both field and class names.
+
+**Example**
+
+.. code-block:: python
+
+    @dataclass
+    class unit:
+        pass
+
+
+    @dataclass
+    class element:
+        unit: Optional[unit] = field()
+
+
+Read :ref:`more <Generator Config>`