diff --git a/csharp/ql/test/query-tests/Nullness/E.cs b/csharp/ql/test/query-tests/Nullness/E.cs
index dcc334b84370..0c020e9b150f 100644
--- a/csharp/ql/test/query-tests/Nullness/E.cs
+++ b/csharp/ql/test/query-tests/Nullness/E.cs
@@ -366,6 +366,16 @@ static void Ex33(string s, object o)
     static int Ex34(string s = null) => s.Length; // BAD (maybe)
 
     static int Ex35(string s = "null") => s.Length; // GOOD
+
+    static int Ex36(object o)
+    {
+        if (o is string)
+        {
+            var s = o as string;
+            return s.Length; // GOOD (false positive)
+        }
+        return -1;
+    }
 }
 
 public static class Extensions
diff --git a/csharp/ql/test/query-tests/Nullness/Implications.expected b/csharp/ql/test/query-tests/Nullness/Implications.expected
index 1119830f7a13..cbb2de7cafcd 100644
--- a/csharp/ql/test/query-tests/Nullness/Implications.expected
+++ b/csharp/ql/test/query-tests/Nullness/Implications.expected
@@ -1231,6 +1231,11 @@
 | E.cs:362:18:362:29 | (...) ... | null | E.cs:362:26:362:29 | null | null |
 | E.cs:363:13:363:13 | access to local variable x | non-null | E.cs:361:17:361:32 | ... ?? ... | non-null |
 | E.cs:363:13:363:13 | access to local variable x | null | E.cs:361:17:361:32 | ... ?? ... | null |
+| E.cs:372:13:372:23 | ... is ... | true | E.cs:372:13:372:13 | access to parameter o | non-null |
+| E.cs:375:20:375:20 | access to local variable s | empty | E.cs:374:21:374:31 | ... as ... | empty |
+| E.cs:375:20:375:20 | access to local variable s | non-empty | E.cs:374:21:374:31 | ... as ... | non-empty |
+| E.cs:375:20:375:20 | access to local variable s | non-null | E.cs:374:21:374:31 | ... as ... | non-null |
+| E.cs:375:20:375:20 | access to local variable s | null | E.cs:374:21:374:31 | ... as ... | null |
 | Forwarding.cs:9:13:9:30 | !... | false | Forwarding.cs:9:14:9:30 | call to method IsNullOrEmpty | true |
 | Forwarding.cs:9:13:9:30 | !... | true | Forwarding.cs:9:14:9:30 | call to method IsNullOrEmpty | false |
 | Forwarding.cs:9:14:9:14 | access to local variable s | empty | Forwarding.cs:7:20:7:23 | null | empty |
diff --git a/csharp/ql/test/query-tests/Nullness/NullCheck.expected b/csharp/ql/test/query-tests/Nullness/NullCheck.expected
index ef3986119d46..d9fcd0121f96 100644
--- a/csharp/ql/test/query-tests/Nullness/NullCheck.expected
+++ b/csharp/ql/test/query-tests/Nullness/NullCheck.expected
@@ -263,6 +263,7 @@
 | E.cs:361:17:361:17 | access to parameter s | E.cs:361:17:361:17 | access to parameter s | null | true |
 | E.cs:362:13:362:29 | ... != ... | E.cs:362:13:362:13 | access to local variable x | false | true |
 | E.cs:362:13:362:29 | ... != ... | E.cs:362:13:362:13 | access to local variable x | true | false |
+| E.cs:372:13:372:23 | ... is ... | E.cs:372:13:372:13 | access to parameter o | true | false |
 | Forwarding.cs:9:14:9:30 | call to method IsNullOrEmpty | Forwarding.cs:9:14:9:14 | access to local variable s | false | false |
 | Forwarding.cs:14:13:14:32 | call to method IsNotNullOrEmpty | Forwarding.cs:14:13:14:13 | access to local variable s | true | false |
 | Forwarding.cs:19:14:19:23 | call to method IsNull | Forwarding.cs:19:14:19:14 | access to local variable s | false | false |
diff --git a/csharp/ql/test/query-tests/Nullness/NullMaybe.expected b/csharp/ql/test/query-tests/Nullness/NullMaybe.expected
index 3842242c80bf..acc3a29817cc 100644
--- a/csharp/ql/test/query-tests/Nullness/NullMaybe.expected
+++ b/csharp/ql/test/query-tests/Nullness/NullMaybe.expected
@@ -353,6 +353,8 @@ nodes
 | E.cs:349:9:349:9 | access to local variable x |
 | E.cs:366:28:366:28 | SSA param(s) |
 | E.cs:366:41:366:41 | access to parameter s |
+| E.cs:374:17:374:31 | SSA def(s) |
+| E.cs:375:20:375:20 | access to local variable s |
 | Forwarding.cs:7:16:7:23 | SSA def(s) |
 | Forwarding.cs:14:9:17:9 | if (...) ... |
 | Forwarding.cs:19:9:22:9 | if (...) ... |
@@ -685,6 +687,7 @@ edges
 | E.cs:342:13:342:32 | SSA def(x) | E.cs:343:9:343:9 | access to local variable x |
 | E.cs:348:17:348:36 | SSA def(x) | E.cs:349:9:349:9 | access to local variable x |
 | E.cs:366:28:366:28 | SSA param(s) | E.cs:366:41:366:41 | access to parameter s |
+| E.cs:374:17:374:31 | SSA def(s) | E.cs:375:20:375:20 | access to local variable s |
 | Forwarding.cs:7:16:7:23 | SSA def(s) | Forwarding.cs:14:9:17:9 | if (...) ... |
 | Forwarding.cs:14:9:17:9 | if (...) ... | Forwarding.cs:19:9:22:9 | if (...) ... |
 | Forwarding.cs:19:9:22:9 | if (...) ... | Forwarding.cs:24:9:27:9 | if (...) ... |
@@ -786,6 +789,7 @@ edges
 | E.cs:343:9:343:9 | access to local variable x | E.cs:342:13:342:32 | SSA def(x) | E.cs:343:9:343:9 | access to local variable x | Variable $@ may be null here because of $@ assignment. | E.cs:342:13:342:13 | x | x | E.cs:342:13:342:32 | String x = ... | this |
 | E.cs:349:9:349:9 | access to local variable x | E.cs:348:17:348:36 | SSA def(x) | E.cs:349:9:349:9 | access to local variable x | Variable $@ may be null here because of $@ assignment. | E.cs:348:17:348:17 | x | x | E.cs:348:17:348:36 | dynamic x = ... | this |
 | E.cs:366:41:366:41 | access to parameter s | E.cs:366:28:366:28 | SSA param(s) | E.cs:366:41:366:41 | access to parameter s | Variable $@ may be null here because the parameter has a null default value. | E.cs:366:28:366:28 | s | s | E.cs:366:32:366:35 | null | this |
+| E.cs:375:20:375:20 | access to local variable s | E.cs:374:17:374:31 | SSA def(s) | E.cs:375:20:375:20 | access to local variable s | Variable $@ may be null here because of $@ assignment. | E.cs:374:17:374:17 | s | s | E.cs:374:17:374:31 | String s = ... | this |
 | GuardedString.cs:35:31:35:31 | access to local variable s | GuardedString.cs:7:16:7:32 | SSA def(s) | GuardedString.cs:35:31:35:31 | access to local variable s | Variable $@ may be null here because of $@ assignment. | GuardedString.cs:7:16:7:16 | s | s | GuardedString.cs:7:16:7:32 | String s = ... | this |
 | NullMaybeBad.cs:7:27:7:27 | access to parameter o | NullMaybeBad.cs:13:17:13:20 | null | NullMaybeBad.cs:7:27:7:27 | access to parameter o | Variable $@ may be null here because of $@ null argument. | NullMaybeBad.cs:5:25:5:25 | o | o | NullMaybeBad.cs:13:17:13:20 | null | this |
 | StringConcatenation.cs:16:17:16:17 | access to local variable s | StringConcatenation.cs:14:16:14:23 | SSA def(s) | StringConcatenation.cs:16:17:16:17 | access to local variable s | Variable $@ may be null here because of $@ assignment. | StringConcatenation.cs:14:16:14:16 | s | s | StringConcatenation.cs:14:16:14:23 | String s = ... | this |