diff --git a/csharp/ql/test/library-tests/dataflow/external-models/ExternalFlow.cs b/csharp/ql/test/library-tests/dataflow/external-models/ExternalFlow.cs
index 7f307e390ea..05772dfb29a 100644
--- a/csharp/ql/test/library-tests/dataflow/external-models/ExternalFlow.cs
+++ b/csharp/ql/test/library-tests/dataflow/external-models/ExternalFlow.cs
@@ -214,4 +214,25 @@ namespace My.Qltest
 
         static void Sink(object o) { }
     }
+
+    public interface HI { }
+
+    public class HC : HI { }
+
+    public static class HE
+    {
+        public static object ExtensionMethod(this HI h) => throw null;
+    }
+
+    public class H
+    {
+        void M1()
+        {
+            var h = new HC();
+            var o = h.ExtensionMethod();
+            Sink(o);
+        }
+
+        static void Sink(object o) { }
+    }
 }
\ No newline at end of file
diff --git a/csharp/ql/test/library-tests/dataflow/external-models/ExternalFlow.expected b/csharp/ql/test/library-tests/dataflow/external-models/ExternalFlow.expected
index 25267c71e87..09ce9945cdf 100644
--- a/csharp/ql/test/library-tests/dataflow/external-models/ExternalFlow.expected
+++ b/csharp/ql/test/library-tests/dataflow/external-models/ExternalFlow.expected
@@ -70,6 +70,9 @@ edges
 | ExternalFlow.cs:197:42:197:43 | access to local variable o2 : Object | ExternalFlow.cs:197:18:197:44 | call to method GeneratedFlowArgs |
 | ExternalFlow.cs:205:22:205:33 | object creation of type Object : Object | ExternalFlow.cs:206:38:206:39 | access to local variable o2 : Object |
 | ExternalFlow.cs:206:38:206:39 | access to local variable o2 : Object | ExternalFlow.cs:206:18:206:40 | call to method MixedFlowArgs |
+| ExternalFlow.cs:231:21:231:28 | object creation of type HC : HC | ExternalFlow.cs:232:21:232:21 | access to local variable h : HC |
+| ExternalFlow.cs:232:21:232:21 | access to local variable h : HC | ExternalFlow.cs:232:21:232:39 | call to method ExtensionMethod : HC |
+| ExternalFlow.cs:232:21:232:39 | call to method ExtensionMethod : HC | ExternalFlow.cs:233:18:233:18 | access to local variable o |
 nodes
 | ExternalFlow.cs:9:27:9:38 | object creation of type Object : Object | semmle.label | object creation of type Object : Object |
 | ExternalFlow.cs:10:18:10:33 | call to method StepArgRes | semmle.label | call to method StepArgRes |
@@ -162,6 +165,10 @@ nodes
 | ExternalFlow.cs:205:22:205:33 | object creation of type Object : Object | semmle.label | object creation of type Object : Object |
 | ExternalFlow.cs:206:18:206:40 | call to method MixedFlowArgs | semmle.label | call to method MixedFlowArgs |
 | ExternalFlow.cs:206:38:206:39 | access to local variable o2 : Object | semmle.label | access to local variable o2 : Object |
+| ExternalFlow.cs:231:21:231:28 | object creation of type HC : HC | semmle.label | object creation of type HC : HC |
+| ExternalFlow.cs:232:21:232:21 | access to local variable h : HC | semmle.label | access to local variable h : HC |
+| ExternalFlow.cs:232:21:232:39 | call to method ExtensionMethod : HC | semmle.label | call to method ExtensionMethod : HC |
+| ExternalFlow.cs:233:18:233:18 | access to local variable o | semmle.label | access to local variable o |
 subpaths
 #select
 | ExternalFlow.cs:10:18:10:33 | call to method StepArgRes | ExternalFlow.cs:9:27:9:38 | object creation of type Object : Object | ExternalFlow.cs:10:18:10:33 | call to method StepArgRes | $@ | ExternalFlow.cs:9:27:9:38 | object creation of type Object : Object | object creation of type Object : Object |
@@ -188,3 +195,4 @@ subpaths
 | ExternalFlow.cs:194:18:194:44 | call to method GeneratedFlowArgs | ExternalFlow.cs:193:22:193:33 | object creation of type Object : Object | ExternalFlow.cs:194:18:194:44 | call to method GeneratedFlowArgs | $@ | ExternalFlow.cs:193:22:193:33 | object creation of type Object : Object | object creation of type Object : Object |
 | ExternalFlow.cs:197:18:197:44 | call to method GeneratedFlowArgs | ExternalFlow.cs:196:22:196:33 | object creation of type Object : Object | ExternalFlow.cs:197:18:197:44 | call to method GeneratedFlowArgs | $@ | ExternalFlow.cs:196:22:196:33 | object creation of type Object : Object | object creation of type Object : Object |
 | ExternalFlow.cs:206:18:206:40 | call to method MixedFlowArgs | ExternalFlow.cs:205:22:205:33 | object creation of type Object : Object | ExternalFlow.cs:206:18:206:40 | call to method MixedFlowArgs | $@ | ExternalFlow.cs:205:22:205:33 | object creation of type Object : Object | object creation of type Object : Object |
+| ExternalFlow.cs:233:18:233:18 | access to local variable o | ExternalFlow.cs:231:21:231:28 | object creation of type HC : HC | ExternalFlow.cs:233:18:233:18 | access to local variable o | $@ | ExternalFlow.cs:231:21:231:28 | object creation of type HC : HC | object creation of type HC : HC |
diff --git a/csharp/ql/test/library-tests/dataflow/external-models/ExternalFlow.ext.yml b/csharp/ql/test/library-tests/dataflow/external-models/ExternalFlow.ext.yml
index 99d387e5ebe..f626949e6f4 100644
--- a/csharp/ql/test/library-tests/dataflow/external-models/ExternalFlow.ext.yml
+++ b/csharp/ql/test/library-tests/dataflow/external-models/ExternalFlow.ext.yml
@@ -29,3 +29,4 @@ extensions:
       - ["My.Qltest", "G", false, "GeneratedFlowArgs", "(System.Object,System.Object)", "", "Argument[1]", "ReturnValue", "value", "df-generated"]
       - ["My.Qltest", "G", false, "MixedFlowArgs", "(System.Object,System.Object)", "", "Argument[0]", "ReturnValue", "value", "df-generated"]
       - ["My.Qltest", "G", false, "MixedFlowArgs", "(System.Object,System.Object)", "", "Argument[1]", "ReturnValue", "value", "manual"]
+      - ["My.Qltest", "HE", false, "ExtensionMethod", "(My.Qltest.HI)", "", "Argument[0]", "ReturnValue", "value", "manual"]