diff --git a/src/BiDaFlow.AsyncEnumerable/Internal/SourceBlockAsyncEnumerable.cs b/src/BiDaFlow.AsyncEnumerable/Internal/SourceBlockAsyncEnumerable.cs
index 75a22e1..1828aa9 100644
--- a/src/BiDaFlow.AsyncEnumerable/Internal/SourceBlockAsyncEnumerable.cs
+++ b/src/BiDaFlow.AsyncEnumerable/Internal/SourceBlockAsyncEnumerable.cs
@@ -107,7 +107,7 @@ public ValueTask DisposeAsync()
 
         Task IDataflowBlock.Completion => throw new NotImplementedException();
 
-        DataflowMessageStatus ITargetBlock<T>.OfferMessage(DataflowMessageHeader messageHeader, T messageValue, ISourceBlock<T> source, bool consumeToAccept)
+        DataflowMessageStatus ITargetBlock<T>.OfferMessage(DataflowMessageHeader messageHeader, T messageValue, ISourceBlock<T>? source, bool consumeToAccept)
         {
             if (source != null && source != this._source) throw new ArgumentException("Unexpected source.");
             if (!messageHeader.IsValid) throw new ArgumentException("messageHeader is not valid.");
diff --git a/src/BiDaFlow/Blocks/EncapsulatingPropagatorBlock.cs b/src/BiDaFlow/Blocks/EncapsulatingPropagatorBlock.cs
index 0b7a716..8642d63 100644
--- a/src/BiDaFlow/Blocks/EncapsulatingPropagatorBlock.cs
+++ b/src/BiDaFlow/Blocks/EncapsulatingPropagatorBlock.cs
@@ -35,8 +35,8 @@ public void ReleaseReservation(DataflowMessageHeader messageHeader, ITargetBlock
         public bool ReserveMessage(DataflowMessageHeader messageHeader, ITargetBlock<TOutput> target)
             => this._terminal.ReserveMessage(messageHeader, target);
 
-        public DataflowMessageStatus OfferMessage(DataflowMessageHeader messageHeader, TInput messageValue, ISourceBlock<TInput> source, bool consumeToAccept)
-            => this._entrance.OfferMessage(messageHeader, messageValue, new ProxySourceBlock<TInput>(this, source), consumeToAccept);
+        public DataflowMessageStatus OfferMessage(DataflowMessageHeader messageHeader, TInput messageValue, ISourceBlock<TInput>? source, bool consumeToAccept)
+            => this._entrance.OfferMessage(messageHeader, messageValue, source != null ? new ProxySourceBlock<TInput>(this, source) : null, consumeToAccept);
 
         public bool TryReceive(Predicate<TOutput>? filter, out TOutput item)
         {
diff --git a/src/BiDaFlow/Blocks/EncapsulatingTargetBlock.cs b/src/BiDaFlow/Blocks/EncapsulatingTargetBlock.cs
index a79dae9..be510c8 100644
--- a/src/BiDaFlow/Blocks/EncapsulatingTargetBlock.cs
+++ b/src/BiDaFlow/Blocks/EncapsulatingTargetBlock.cs
@@ -21,7 +21,7 @@ public EncapsulatingTargetBlock(ITargetBlock<T> entrance, IDataflowBlock termina
 
         public void Fault(Exception exception) => this._entrance.Fault(exception);
 
-        public DataflowMessageStatus OfferMessage(DataflowMessageHeader messageHeader, T messageValue, ISourceBlock<T> source, bool consumeToAccept)
-            => this._entrance.OfferMessage(messageHeader, messageValue, new ProxySourceBlock<T>(this, source), consumeToAccept);
+        public DataflowMessageStatus OfferMessage(DataflowMessageHeader messageHeader, T messageValue, ISourceBlock<T>? source, bool consumeToAccept)
+            => this._entrance.OfferMessage(messageHeader, messageValue, source != null ? new ProxySourceBlock<T>(this, source) : null, consumeToAccept);
     }
 }
diff --git a/src/BiDaFlow/Blocks/ProxySourceBlock.cs b/src/BiDaFlow/Blocks/ProxySourceBlock.cs
index 8c148b5..4488062 100644
--- a/src/BiDaFlow/Blocks/ProxySourceBlock.cs
+++ b/src/BiDaFlow/Blocks/ProxySourceBlock.cs
@@ -17,6 +17,9 @@ internal sealed class ProxySourceBlock<T> : ISourceBlock<T>
 
         public ProxySourceBlock(ITargetBlock<T> targetWrapper, ISourceBlock<T> source)
         {
+            if (targetWrapper == null) throw new ArgumentNullException(nameof(targetWrapper));
+            if (source == null) throw new ArgumentNullException(nameof(source));
+
             this._targetWrapper = targetWrapper;
             this._source = source;
         }
diff --git a/src/BiDaFlow/CHANGELOG.md b/src/BiDaFlow/CHANGELOG.md
index e77e320..4589eb3 100644
--- a/src/BiDaFlow/CHANGELOG.md
+++ b/src/BiDaFlow/CHANGELOG.md
@@ -4,6 +4,10 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## Unreleased
+### Fixed
+- Fix wrong null handling in the classes encapsulating `ITargetBlock`
+
 ## [0.2.0-alpha1] - 2020-10-13
 ### Added
 - Add simple encapulation methods