diff --git a/gclient.py b/gclient.py
index b25dfaa31..0481644ce 100755
--- a/gclient.py
+++ b/gclient.py
@@ -567,15 +567,19 @@ class Dependency(gclient_utils.WorkItem, DependencySettings):
         target_os_deps[os_dep_key] = None
       else:
         if len(possible_values) > 1:
-          # It would be possible to abort here but it would be
-          # unfortunate if we end up preventing any kind of checkout.
-          logging.error('Conflicting dependencies for %s: %s. (target_os=%s)',
-                        os_dep_key, os_dep_value, target_os_list)
+          raise gclient_utils.Error(
+              'Conflicting dependencies for %s: %s. (target_os=%s)' % (
+                  os_dep_key, os_dep_value, target_os_list))
         # Sorting to get the same result every time in case of conflicts.
         target_os_deps[os_dep_key] = sorted(possible_values)[0]
 
     new_deps = deps.copy()
-    new_deps.update(target_os_deps)
+    for key, value in target_os_deps.iteritems():
+      if key in new_deps:
+        raise gclient_utils.Error(
+            ('Value from deps_os (%r: %r) conflicts with existing deps '
+             'entry (%r).') % (key, value, new_deps[key]))
+      new_deps[key] = value
     return new_deps
 
   def _postprocess_deps(self, deps, rel_prefix):
diff --git a/tests/gclient_test.py b/tests/gclient_test.py
index baa3d377f..7c19c3dc4 100755
--- a/tests/gclient_test.py
+++ b/tests/gclient_test.py
@@ -535,20 +535,6 @@ class GclientTest(trial_dir.TestCase):
 
     test_data = [
       # Tuples of deps, deps_os, os_list and expected_deps.
-      (
-        # OS doesn't need module.
-        {'foo': 'default_foo'},
-        {'os1': { 'foo': None } },
-        ['os1'],
-        {'foo': None}
-        ),
-      (
-        # OS wants a different version of module.
-        {'foo': 'default_foo'},
-        {'os1': { 'foo': 'os1_foo'} },
-        ['os1'],
-        {'foo': 'os1_foo'}
-        ),
       (
         # OS with no overrides at all.
         {'foo': 'default_foo'},
@@ -556,22 +542,6 @@ class GclientTest(trial_dir.TestCase):
         ['os2'],
         {'foo': 'default_foo'}
         ),
-      (
-        # One OS doesn't need module, one OS wants the default.
-        {'foo': 'default_foo'},
-        {'os1': { 'foo': None },
-         'os2': {}},
-        ['os1', 'os2'],
-        {'foo': 'default_foo'}
-        ),
-      (
-        # One OS doesn't need module, another OS wants a special version.
-        {'foo': 'default_foo'},
-        {'os1': { 'foo': None },
-         'os2': { 'foo': 'os2_foo'}},
-        ['os1', 'os2'],
-        {'foo': 'os2_foo'}
-        ),
       (
         # One OS wants to add a module.
         {'foo': 'default_foo'},
@@ -604,6 +574,39 @@ class GclientTest(trial_dir.TestCase):
       self.assertEqual(result, expected_deps)
       self.assertEqual(deps, orig_deps)
 
+  def testUpdateWithOsDepsInvalid(self):
+    test_data = [
+      # Tuples of deps, deps_os, os_list.
+      (
+        # OS doesn't need module.
+        {'foo': 'default_foo'},
+        {'os1': { 'foo': None } },
+        ['os1'],
+        ),
+      (
+        # OS wants a different version of module.
+        {'foo': 'default_foo'},
+        {'os1': { 'foo': 'os1_foo'} },
+        ['os1'],
+        ),
+      (
+        # One OS doesn't need module, one OS wants the default.
+        {'foo': 'default_foo'},
+        {'os1': { 'foo': None },
+         'os2': {}},
+        ['os1', 'os2'],
+        ),
+      (
+        # One OS doesn't need module, another OS wants a special version.
+        {'foo': 'default_foo'},
+        {'os1': { 'foo': None },
+         'os2': { 'foo': 'os2_foo'}},
+        ['os1', 'os2'],
+        ),
+      ]
+    for deps, deps_os, target_os_list in test_data:
+      with self.assertRaises(gclient_utils.Error):
+        gclient.Dependency.MergeWithOsDeps(deps, deps_os, target_os_list)
 
   def testLateOverride(self):
     """Verifies expected behavior of LateOverride."""
@@ -614,7 +617,7 @@ class GclientTest(trial_dir.TestCase):
     self.assertEquals(url, late_url)
 
   def testDepsOsOverrideDepsInDepsFile(self):
-    """Verifies that a 'deps_os' path can override a 'deps' path. Also
+    """Verifies that a 'deps_os' path cannot override a 'deps' path. Also
     see testUpdateWithOsDeps above.
     """
 
@@ -644,14 +647,12 @@ class GclientTest(trial_dir.TestCase):
     options.deps_os = 'unix'
 
     obj = gclient.GClient.LoadCurrentConfig(options)
-    obj.RunOnDeps('None', [])
+    with self.assertRaises(gclient_utils.Error):
+      obj.RunOnDeps('None', [])
     self.assertEqual(['unix'], sorted(obj.enforced_os))
     self.assertEquals(
         [
           ('foo', 'svn://example.com/foo'),
-          ('foo/baz', 'svn://example.com/foo/baz'),
-          ('foo/src', 'svn://example.com/foo/src_unix'),
-          ('foo/unix', 'svn://example.com/foo/unix'),
           ],
         sorted(self._get_processed()))