diff --git a/src/tyro/constructors/_primitive_spec.py b/src/tyro/constructors/_primitive_spec.py
index 80575e37..729c2999 100644
--- a/src/tyro/constructors/_primitive_spec.py
+++ b/src/tyro/constructors/_primitive_spec.py
@@ -142,7 +142,10 @@ def basics_rule(type_info: PrimitiveTypeInfo) -> PrimitiveConstructorSpec | None
                 if type_info.type is bytes
                 else type_info.type(args[0])
             ),
-            is_instance=lambda x: isinstance(x, type_info.type),
+            # Numeric tower in Python is weird...
+            is_instance=lambda x: isinstance(x, (int, float))
+            if type_info.type is float
+            else isinstance(x, type_info.type),
             str_from_instance=lambda instance: [str(instance)],
         )
 
@@ -502,13 +505,11 @@ def instance_from_str(args: list[str]) -> dict:
             return out
 
         def str_from_instance(instance: dict) -> list[str]:
+            # TODO: this may be strange right now for the append action.
             out: list[str] = []
-            assert (
-                len(instance) == 0
-            ), "When parsed as a primitive, we currrently assume all defaults are length=0. Dictionaries with non-zero-length defaults are interpreted as struct types."
-            # for key, value in instance.items():
-            #     out.extend(key_spec.str_from_instance(key))
-            #     out.extend(val_spec.str_from_instance(value))
+            for key, value in instance.items():
+                out.extend(key_spec.str_from_instance(key))
+                out.extend(val_spec.str_from_instance(value))
             return out
 
         if _markers.UseAppendAction in type_info.markers:
diff --git a/src/tyro/constructors/_struct_spec.py b/src/tyro/constructors/_struct_spec.py
index 6a9aeecd..06f0776f 100644
--- a/src/tyro/constructors/_struct_spec.py
+++ b/src/tyro/constructors/_struct_spec.py
@@ -274,6 +274,7 @@ def dict_rule(info: StructTypeInfo) -> StructConstructorSpec | None:
         if is_typeddict(info.type) or (
             info.type
             not in (
+                Dict,
                 dict,
                 collections.abc.Mapping,
             )
diff --git a/tests/test_collections.py b/tests/test_collections.py
index 2f84ffef..300d1481 100644
--- a/tests/test_collections.py
+++ b/tests/test_collections.py
@@ -482,6 +482,26 @@ def main(x: Dict[str, Any] = {"int": 5, "str": "5"}):
     }
 
 
+def test_dict_no_annotation_2() -> None:
+    def main(x: Dict = {"int": 5, "str": "5"}):
+        return x
+
+    assert tyro.cli(main, args=[]) == {"int": 5, "str": "5"}
+    assert tyro.cli(main, args="--x.int 3 --x.str 7".split(" ")) == {
+        "int": 3,
+        "str": "7",
+    }
+
+
+def test_dict_optional() -> None:
+    # In this case, the `None` is ignored.
+    def main(x: Optional[Dict[str, float]] = {"three": 3, "five": 5}):
+        return x
+
+    assert tyro.cli(main, args=[]) == {"three": 3, "five": 5}
+    assert tyro.cli(main, args="--x 3 3 5 5".split(" ")) == {"3": 3, "5": 5}
+
+
 def test_double_dict_no_annotation() -> None:
     def main(
         x: Dict[str, Any] = {
diff --git a/tests/test_py311_generated/test_collections_generated.py b/tests/test_py311_generated/test_collections_generated.py
index 5b6e3d81..9aca7e8a 100644
--- a/tests/test_py311_generated/test_collections_generated.py
+++ b/tests/test_py311_generated/test_collections_generated.py
@@ -481,6 +481,26 @@ def main(x: Dict[str, Any] = {"int": 5, "str": "5"}):
     }
 
 
+def test_dict_no_annotation_2() -> None:
+    def main(x: Dict = {"int": 5, "str": "5"}):
+        return x
+
+    assert tyro.cli(main, args=[]) == {"int": 5, "str": "5"}
+    assert tyro.cli(main, args="--x.int 3 --x.str 7".split(" ")) == {
+        "int": 3,
+        "str": "7",
+    }
+
+
+def test_dict_optional() -> None:
+    # In this case, the `None` is ignored.
+    def main(x: Optional[Dict[str, int]] = {"three": 3, "five": 5}):
+        return x
+
+    assert tyro.cli(main, args=[]) == {"three": 3, "five": 5}
+    assert tyro.cli(main, args="--x 3 3 5 5".split(" ")) == {"3": 3, "5": 5}
+
+
 def test_double_dict_no_annotation() -> None:
     def main(
         x: Dict[str, Any] = {