diff --git a/dataclasses_json/core.py b/dataclasses_json/core.py
index 3dafe9f9..3b755321 100644
--- a/dataclasses_json/core.py
+++ b/dataclasses_json/core.py
@@ -314,17 +314,17 @@ def _decode_generic(type_, value, infer_missing):
             type_options = _get_type_args(type_)
             res = value  # assume already decoded
             if type(value) is dict and dict not in type_options:
-                # FIXME if all types in the union are dataclasses this
-                #  will just pick the first option -
-                #  maybe find the best fitting class in that case instead?
                 for type_option in type_options:
                     if is_dataclass(type_option):
-                        res = _decode_dataclass(type_option, value, infer_missing)
-                        break
+                        try:
+                            res = _decode_dataclass(type_option, value, infer_missing)
+                            break
+                        except (KeyError, ValueError):
+                            continue
                 if res == value:
                     warnings.warn(
-                        f"Failed to encode {value} Union dataclasses."
-                        f"Expected Union to include a dataclass and it didn't."
+                        f"Failed to decode {value} Union dataclasses."
+                        f"Expected Union to include a matching dataclass and it didn't."
                     )
     return res
 
diff --git a/dataclasses_json/mm.py b/dataclasses_json/mm.py
index 8e889222..9cfacf1d 100644
--- a/dataclasses_json/mm.py
+++ b/dataclasses_json/mm.py
@@ -100,16 +100,25 @@ def _deserialize(self, value, attr, data, **kwargs):
                 if is_dataclass(type_) and type_.__name__ == dc_name:
                     del tmp_value['__type']
                     return schema_._deserialize(tmp_value, attr, data, **kwargs)
-        for type_, schema_ in self.desc.items():
-            if isinstance(tmp_value, _get_type_origin(type_)):
-                return schema_._deserialize(tmp_value, attr, data, **kwargs)
-        else:
+        elif isinstance(tmp_value, dict):
             warnings.warn(
-                f'The type "{type(tmp_value).__name__}" (value: "{tmp_value}") '
-                f'is not in the list of possible types of typing.Union '
-                f'(dataclass: {self.cls.__name__}, field: {self.field.name}). '
-                f'Value cannot be deserialized properly.')
-        return super()._deserialize(tmp_value, attr, data, **kwargs)
+                f'Attempting to deserialize "dict" (value: "{tmp_value}) '
+                f'that does not have a "__type" type specifier field into'
+                f'(dataclass: {self.cls.__name__}, field: {self.field.name}).'
+                f'Deserialization may fail, or deserialization to wrong type may occur.'
+            )
+            return super()._deserialize(tmp_value, attr, data, **kwargs)
+        else:
+            for type_, schema_ in self.desc.items():
+                if isinstance(tmp_value, _get_type_origin(type_)):
+                    return schema_._deserialize(tmp_value, attr, data, **kwargs)
+            else:
+                warnings.warn(
+                    f'The type "{type(tmp_value).__name__}" (value: "{tmp_value}") '
+                    f'is not in the list of possible types of typing.Union '
+                    f'(dataclass: {self.cls.__name__}, field: {self.field.name}). '
+                    f'Value cannot be deserialized properly.')
+            return super()._deserialize(tmp_value, attr, data, **kwargs)
 
 
 class _TupleVarLen(fields.List):
diff --git a/tests/test_union.py b/tests/test_union.py
index d02f8d1b..57b848b9 100644
--- a/tests/test_union.py
+++ b/tests/test_union.py
@@ -37,11 +37,21 @@ class Aux2:
     f1: str
 
 
+@dataclass_json
+@dataclass
+class Aux3:
+    f2: str
+
 @dataclass_json
 @dataclass
 class C4:
     f1: Union[Aux1, Aux2]
 
+@dataclass_json
+@dataclass
+class C12:
+    f1: Union[Aux2, Aux3]
+
 
 @dataclass_json
 @dataclass
@@ -198,3 +208,35 @@ def test_deserialize_with_error(cls, data):
     s = cls.schema()
     with pytest.raises(ValidationError):
         assert s.load(data)
+
+def test_deserialize_without_discriminator():
+    # determine based on type
+    json = '{"f1": {"f1": 1}}'
+    s = C4.schema()
+    obj = s.loads(json)
+    assert obj.f1 is not None
+    assert type(obj.f1) == Aux1
+
+    json = '{"f1": {"f1": "str1"}}'
+    s = C4.schema()
+    obj = s.loads(json)
+    assert obj.f1 is not None
+    assert type(obj.f1) == Aux2
+
+    # determine based on field name
+    json = '{"f1": {"f1": "str1"}}'
+    s = C12.schema()
+    obj = s.loads(json)
+    assert obj.f1 is not None
+    assert type(obj.f1) == Aux2
+    json = '{"f1": {"f2": "str1"}}'
+    s = C12.schema()
+    obj = s.loads(json)
+    assert obj.f1 is not None
+    assert type(obj.f1) == Aux3
+
+    # if no matching types, type should remain dict
+    json = '{"f1": {"f3": "str2"}}'
+    s = C12.schema()
+    obj = s.loads(json)
+    assert type(obj.f1) == dict
\ No newline at end of file