diff --git a/confuse/util.py b/confuse/util.py
index f8ffc34..564744e 100644
--- a/confuse/util.py
+++ b/confuse/util.py
@@ -112,7 +112,7 @@ def build_dict(
                         save_to = save_to[child_key]
 
         # Save
-        if key in save_to:
+        if isinstance(save_to.get(key), dict) and isinstance(result, dict):
             save_to[key].update(result)
         else:
             save_to[key] = result
diff --git a/docs/changelog.rst b/docs/changelog.rst
index 680346e..71decd4 100644
--- a/docs/changelog.rst
+++ b/docs/changelog.rst
@@ -13,9 +13,13 @@ Unreleased
     New features
     ~~~~~~~~~~~~
 
-..
-    Bug fixes
-    ~~~~~~~~~
+Bug fixes
+~~~~~~~~~
+
+- Fix nested environment and command-line overrides colliding with values in
+  parent mappings. More-specific values replace scalar, list, or null values;
+  mappings continue to merge. `#215
+  <https://github.com/beetbox/confuse/issues/215>`_
 
 ..
     For plugin developers
diff --git a/test/test_env.py b/test/test_env.py
index fe476a8..64cc958 100644
--- a/test/test_env.py
+++ b/test/test_env.py
@@ -270,3 +270,34 @@ def test_with_sep(self):
     def test_with_prefix_and_sep(self):
         self.config.set_env(prefix="MYAPP_", sep="_sep_")
         assert self.config.get() == {"qux": {"nested": "d"}}
+
+
+@pytest.mark.parametrize("reverse", [False, True])
+@pytest.mark.parametrize(
+    "original",
+    ["5432", "localhost", "[5432, 5434]", "null", "{fallback: 5432}"],
+)
+@pytest.mark.parametrize(
+    "override, expected",
+    [
+        ("5433", 5433),
+        ("remote", "remote"),
+        ("[5433, 5435]", [5433, 5435]),
+        ("null", None),
+        ("{primary: 5433}", {"primary": 5433}),
+    ],
+)
+def test_yaml_nested_leaf_override(original, override, expected, reverse):
+    entries = [
+        ("TEST_DATABASE", f"{{host: localhost, port: {original}}}"),
+        ("TEST_DATABASE__PORT", override),
+    ]
+    if reverse:
+        entries.reverse()
+    environment = dict(entries)
+    with patch.dict("os.environ", environment, clear=True):
+        source = confuse.EnvSource("TEST_", parse_yaml_docs=True)
+        assert dict(os.environ) == environment
+    if original == "{fallback: 5432}" and isinstance(expected, dict):
+        expected = {"fallback": 5432, **expected}
+    assert source["database"] == {"host": "localhost", "port": expected}
diff --git a/test/test_utils.py b/test/test_utils.py
index 176fcdf..367c70f 100644
--- a/test/test_utils.py
+++ b/test/test_utils.py
@@ -1,6 +1,7 @@
 import unittest
 from argparse import Namespace
 from collections import OrderedDict
+from copy import deepcopy
 from typing import Any
 
 import pytest
@@ -81,3 +82,50 @@ def test_keep_none_with_nested(self):
 
         result = confuse.util.build_dict(config.copy(), keep_none=True)
         assert None is result["foo"]["bar"]
+
+
+@pytest.mark.parametrize("reverse", [False, True])
+@pytest.mark.parametrize("namespace", [False, True])
+@pytest.mark.parametrize("original", [5432, "local", [5432], None, {"fallback": 5432}])
+@pytest.mark.parametrize("override", [5433, "remote", [5433], {"primary": 5433}])
+def test_set_args_nested_leaf_override(original, override, namespace, reverse):
+    entries = [
+        ("database", {"host": "localhost", "port": original}),
+        ("database.port", override),
+    ]
+    if reverse:
+        entries.reverse()
+    args: dict[str, Any] | Namespace = dict(entries)
+    if namespace:
+        args = Namespace(**args)
+    before = deepcopy(args)
+    config = confuse.Configuration("test", read=False)
+    config.set_args(args, dots=True)
+    expected = override
+    if isinstance(original, dict) and isinstance(override, dict):
+        expected = {**original, **override}
+    assert config["database"].get() == {"host": "localhost", "port": expected}
+    assert args == before
+
+
+@pytest.mark.parametrize("reverse", [False, True])
+def test_set_args_none_override_remains_ignored(reverse):
+    entries = [("database", {"port": 5432}), ("database.port", None)]
+    if reverse:
+        entries.reverse()
+    config = confuse.Configuration("test", read=False)
+    config.set_args(dict(entries), dots=True)
+    assert config["database"]["port"].get() == 5432
+
+
+def test_build_dict_mapping_merge_retains_order_and_is_shallow():
+    args: dict[str, Any] = {
+        "database": {"options": {"first": 1, "nested": {"old": 2}}},
+        "database.options": {"nested": {"new": 3}, "last": 4},
+    }
+    result = confuse.util.build_dict(args, sep=".")
+    assert result == {
+        "database": {"options": {"first": 1, "nested": {"new": 3}, "last": 4}}
+    }
+    assert list(result["database"]["options"]) == ["first", "nested", "last"]
+    assert args["database"]["options"]["nested"] == {"old": 2}