diff --git a/csharp/src/Google.Protobuf.Test/GeneratedMessageTest.Proto2.cs b/csharp/src/Google.Protobuf.Test/GeneratedMessageTest.Proto2.cs
index 1abed60563c8..894d91412493 100644
--- a/csharp/src/Google.Protobuf.Test/GeneratedMessageTest.Proto2.cs
+++ b/csharp/src/Google.Protobuf.Test/GeneratedMessageTest.Proto2.cs
@@ -380,5 +380,18 @@ public void RoundTrip_NestedExtensionGroup()
                 TestGroupExtension.Parser.WithExtensionRegistry(new ExtensionRegistry() { TestNestedExtension.Extensions.OptionalGroupExtension }),
                 message);
         }
+
+        [Test]
+        public void RoundTrip_ParseUsingCodedInput()
+        {
+            var message = new TestAllExtensions();
+            message.SetExtension(UnittestExtensions.OptionalBoolExtension, true);
+            byte[] bytes = message.ToByteArray();
+            using (CodedInputStream input = new CodedInputStream(bytes))
+            {
+                var parsed = TestAllExtensions.Parser.WithExtensionRegistry(new ExtensionRegistry() { UnittestExtensions.OptionalBoolExtension }).ParseFrom(input);
+                Assert.AreEqual(message, parsed);
+            }
+        }
     }
 }
diff --git a/csharp/src/Google.Protobuf/MessageParser.cs b/csharp/src/Google.Protobuf/MessageParser.cs
index 30a25a8698d3..a10c90891640 100644
--- a/csharp/src/Google.Protobuf/MessageParser.cs
+++ b/csharp/src/Google.Protobuf/MessageParser.cs
@@ -187,14 +187,17 @@ public IMessage ParseJson(string json)
         internal void MergeFrom(IMessage message, CodedInputStream codedInput)
         {
             bool originalDiscard = codedInput.DiscardUnknownFields;
+            ExtensionRegistry originalRegistry = codedInput.ExtensionRegistry;
             try
             {
                 codedInput.DiscardUnknownFields = DiscardUnknownFields;
+                codedInput.ExtensionRegistry = Extensions;
                 message.MergeFrom(codedInput);
             }
             finally
             {
                 codedInput.DiscardUnknownFields = originalDiscard;
+                codedInput.ExtensionRegistry = originalRegistry;
             }
         }