diff --git a/qutebrowser/config/configtypes.py b/qutebrowser/config/configtypes.py
index 14c4849b7..b8c9c1e05 100644
--- a/qutebrowser/config/configtypes.py
+++ b/qutebrowser/config/configtypes.py
@@ -510,7 +510,7 @@ class FlagList(List):
         # Single value completions
         for value in valid_values:
             desc = valid_values.descriptions.get(value, "")
-            out.append((value, desc))
+            out.append((json.dumps([value]), desc))
 
         combinables = self.combinable_values
         if combinables is None:
@@ -518,7 +518,7 @@ class FlagList(List):
         # Generate combinations of each possible value combination
         for size in range(2, len(combinables) + 1):
             for combination in itertools.combinations(combinables, size):
-                out.append((','.join(combination), ''))
+                out.append((json.dumps(combination), ''))
         return out
 
 
diff --git a/tests/unit/config/test_configtypes.py b/tests/unit/config/test_configtypes.py
index 5daf63294..d9845ba03 100644
--- a/tests/unit/config/test_configtypes.py
+++ b/tests/unit/config/test_configtypes.py
@@ -278,6 +278,14 @@ class TestAll:
         with pytest.raises(configexc.ValidationError):
             klass().to_py(object())
 
+    def test_completion_validity(self, klass):
+        """Make sure all completions are actually valid values."""
+        typ = klass()
+        completions = typ.complete()
+        if completions is not None:
+            for value, _desc in completions:
+                typ.from_str(value)
+
 
 class TestBaseType:
 
@@ -640,24 +648,24 @@ class TestFlagList:
     def test_complete(self, klass):
         """Test completing by doing some samples."""
         typ = klass(set_valid_values=True)
-        completions = [e[0] for e in typ.complete()]
-        assert 'foo' in completions
-        assert 'bar' in completions
-        assert 'baz' in completions
-        assert 'foo,bar' in completions
+        completions = [json.loads(e[0]) for e in typ.complete()]
+        assert ['foo'] in completions
+        assert ['bar'] in completions
+        assert ['baz'] in completions
+        assert ['foo', 'bar'] in completions
         for val in completions:
-            assert 'baz,' not in val
-            assert ',baz' not in val
+            if len(val) > 1:
+                assert 'baz' not in val
 
     def test_complete_all_valid_values(self, klass):
         typ = klass(set_valid_values=True)
         typ.combinable_values = None
-        completions = [e[0] for e in typ.complete()]
-        assert 'foo' in completions
-        assert 'bar' in completions
-        assert 'baz' in completions
-        assert 'foo,bar' in completions
-        assert 'foo,baz' in completions
+        completions = [json.loads(e[0]) for e in typ.complete()]
+        assert ['foo'] in completions
+        assert ['bar'] in completions
+        assert ['baz'] in completions
+        assert ['foo', 'bar'] in completions
+        assert ['foo', 'baz'] in completions
 
     def test_complete_no_valid_values(self, klass):
         assert klass().complete() is None
@@ -1910,18 +1918,6 @@ class TestConfirmQuit:
         with pytest.raises(configexc.ValidationError):
             klass().to_py(val)
 
-    def test_complete(self, klass):
-        """Test completing by doing some samples."""
-        completions = [e[0] for e in klass().complete()]
-        assert 'always' in completions
-        assert 'never' in completions
-        assert 'multiple-tabs,downloads' in completions
-        for val in completions:
-            assert 'always,' not in val
-            assert ',always' not in val
-            assert 'never,' not in val
-            assert ',never' not in val
-
 
 class TestTimestampTemplate: