diff --git a/CHANGELOG.md b/CHANGELOG.md
index 5060cf61..23047a9f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -13,6 +13,7 @@ All notable changes to this project will be documented in this file.
 - API: don't let SuspiciousOperation bubble up when validating channel ids
 - API security: check channel ownership when setting check's channels
 - API: update check's "alert_after" field when changing schedule
+- API: validate channel identifiers before creating/updating a check (#335)
 
 ## v1.13.0 - 2020-02-13
 
diff --git a/hc/api/tests/test_create_check.py b/hc/api/tests/test_create_check.py
index 9dd80b59..9f8e3d96 100644
--- a/hc/api/tests/test_create_check.py
+++ b/hc/api/tests/test_create_check.py
@@ -84,6 +84,14 @@ class CreateCheckTestCase(BaseTestCase):
         check = Check.objects.get()
         self.assertEqual(check.channel_set.get(), channel)
 
+    def test_it_rejects_bad_channel_code(self):
+        r = self.post({"api_key": "X" * 32, "channels": "abc"})
+        self.assertEqual(r.status_code, 400)
+        self.assertEqual(r.json()["error"], "invalid channel identifier: abc")
+
+        # The check should not have been saved
+        self.assertFalse(Check.objects.exists())
+
     def test_it_supports_unique(self):
         Check.objects.create(project=self.project, name="Foo")
 
diff --git a/hc/api/tests/test_update_check.py b/hc/api/tests/test_update_check.py
index f689f193..a46eec39 100644
--- a/hc/api/tests/test_update_check.py
+++ b/hc/api/tests/test_update_check.py
@@ -119,6 +119,15 @@ class UpdateCheckTestCase(BaseTestCase):
         self.assertEqual(self.check.channel_set.count(), 1)
         self.assertEqual(self.check.channel_set.first().code, channel.code)
 
+    def test_it_sets_the_channel_only_once(self):
+        channel = Channel.objects.create(project=self.project)
+        duplicates = "%s,%s" % (channel.code, channel.code)
+        r = self.post(self.check.code, {"api_key": "X" * 32, "channels": duplicates})
+        self.assertEqual(r.status_code, 200)
+
+        self.check.refresh_from_db()
+        self.assertEqual(self.check.channel_set.count(), 1)
+
     def test_it_handles_comma_separated_channel_codes(self):
         c1 = Channel.objects.create(project=self.project)
         c2 = Channel.objects.create(project=self.project)
@@ -152,10 +161,15 @@ class UpdateCheckTestCase(BaseTestCase):
         self.assertEqual(check.channel_set.count(), 1)
 
     def test_it_rejects_bad_channel_code(self):
-        r = self.post(self.check.code, {"api_key": "X" * 32, "channels": "abc"})
+        payload = {"api_key": "X" * 32, "channels": "abc", "name": "New Name"}
+        r = self.post(self.check.code, payload,)
         self.assertEqual(r.status_code, 400)
         self.assertEqual(r.json()["error"], "invalid channel identifier: abc")
 
+        # The name should be unchanged
+        self.check.refresh_from_db()
+        self.assertEqual(self.check.name, "")
+
     def test_it_rejects_missing_channel(self):
         code = str(uuid.uuid4())
         r = self.post(self.check.code, {"api_key": "X" * 32, "channels": code})
diff --git a/hc/api/views.py b/hc/api/views.py
index abcb67a8..bc675d57 100644
--- a/hc/api/views.py
+++ b/hc/api/views.py
@@ -67,6 +67,21 @@ def _lookup(project, spec):
 
 
 def _update(check, spec):
+    channels = set()
+    # First, validate the supplied channel codes
+    if "channels" in spec and spec["channels"] not in ("*", ""):
+        q = Channel.objects.filter(project=check.project)
+        for s in spec["channels"].split(","):
+            try:
+                code = uuid.UUID(s)
+            except ValueError:
+                raise BadChannelException("invalid channel identifier: %s" % s)
+
+            try:
+                channels.add(q.get(code=code))
+            except Channel.DoesNotExist:
+                raise BadChannelException("invalid channel identifier: %s" % s)
+
     if "name" in spec:
         check.name = spec["name"]
 
@@ -94,26 +109,12 @@ def _update(check, spec):
 
     # This needs to be done after saving the check, because of
     # the M2M relation between checks and channels:
-    if "channels" in spec:
-        if spec["channels"] == "*":
-            check.assign_all_channels()
-        elif spec["channels"] == "":
-            check.channel_set.clear()
-        else:
-            channels = []
-            channel_query = Channel.objects.filter(project=check.project)
-            for chunk in spec["channels"].split(","):
-                try:
-                    chunk = uuid.UUID(chunk)
-                except ValueError:
-                    raise BadChannelException("invalid channel identifier: %s" % chunk)
-
-                try:
-                    channels.append(channel_query.get(code=chunk))
-                except Channel.DoesNotExist:
-                    raise BadChannelException("invalid channel identifier: %s" % chunk)
-
-            check.channel_set.set(channels)
+    if spec.get("channels") == "*":
+        check.assign_all_channels()
+    elif spec.get("channels") == "":
+        check.channel_set.clear()
+    elif channels:
+        check.channel_set.set(channels)
 
     return check