diff --git a/Code/GraphMol/FileParsers/MultithreadedMolSupplier.cpp b/Code/GraphMol/FileParsers/MultithreadedMolSupplier.cpp
index b33842ca4..63b56858c 100644
--- a/Code/GraphMol/FileParsers/MultithreadedMolSupplier.cpp
+++ b/Code/GraphMol/FileParsers/MultithreadedMolSupplier.cpp
@@ -81,6 +81,7 @@ void MultithreadedMolSupplier::reader() {
     auto r = std::make_tuple(record, lineNum, index);
     if (!df_forceStop) {
       d_inputQueue->push(r);
+      ++d_mols_read;
     }
   }
   d_inputQueue->setDone();
@@ -106,23 +107,6 @@ void MultithreadedMolSupplier::writer() {
       d_outputQueue->push(nullValue);
     }
   }
-
-  // we need a lock here otherwise two threads
-  //  can increment d_threadCounter even though it's
-  //  atomic.
-  d_threadCounterMutex.lock();
-  if (d_threadCounter < d_params.numWriterThreads) {
-    ++d_threadCounter;
-    d_threadCounterMutex.unlock();
-  } else {
-    // Here we need to unlock the threadCounterMutex before we setDone on the
-    //  outputQueue.  This causes a notification to the queue which may actually
-    //  have elements in it.  This notification may unblock the queue which
-    //  allows waiting threads to get their last attempt at adding to it
-    //  which will end up here and deadlock.
-    d_threadCounterMutex.unlock();
-    d_outputQueue->setDone();
-  }
 }
 
 std::unique_ptr<RWMol> MultithreadedMolSupplier::next() {
@@ -131,7 +115,11 @@ std::unique_ptr<RWMol> MultithreadedMolSupplier::next() {
     startThreads();
   }
   std::tuple<RWMol *, std::string, unsigned int> r;
-  if (!df_forceStop && d_outputQueue->pop(r)) {
+  if (!df_forceStop) {
+    if (!d_outputQueue->pop(r)) {
+      throw FileParseException("something went wrong");
+    }
+
     d_lastItemText = std::get<1>(r);
     d_lastRecordId = std::get<2>(r);
     std::unique_ptr<RWMol> res{std::get<0>(r)};
@@ -142,9 +130,10 @@ std::unique_ptr<RWMol> MultithreadedMolSupplier::next() {
         // Ignore exception and proceed with mol as is.
       }
     }
+    ++d_mols_returned;
     return res;
   }
-  return nullptr;
+  throw FileParseException("stop forced");
 }
 
 // this calls joins on the reader and writer threads
@@ -164,9 +153,9 @@ void MultithreadedMolSupplier::endThreads() {
 }
 
 void MultithreadedMolSupplier::startThreads() {
-  // run the reader function in a seperate thread
+  // run the reader function in a separate thread
   d_readerThread = std::thread(&MultithreadedMolSupplier::reader, this);
-  // run the writer function in seperate threads
+  // run the writer function in separate threads
   for (unsigned int i = 0; i < d_params.numWriterThreads; i++) {
     d_writerThreads.emplace_back(
         std::thread(&MultithreadedMolSupplier::writer, this));
@@ -174,7 +163,7 @@ void MultithreadedMolSupplier::startThreads() {
 }
 
 bool MultithreadedMolSupplier::atEnd() {
-  return (d_outputQueue->isEmpty() && d_outputQueue->getDone());
+  return (d_inputQueue->getDone() && d_mols_read == d_mols_returned);
 }
 
 unsigned int MultithreadedMolSupplier::getLastRecordId() const {
diff --git a/Code/GraphMol/FileParsers/MultithreadedMolSupplier.h b/Code/GraphMol/FileParsers/MultithreadedMolSupplier.h
index 4e91e0cf4..8bf09c51e 100644
--- a/Code/GraphMol/FileParsers/MultithreadedMolSupplier.h
+++ b/Code/GraphMol/FileParsers/MultithreadedMolSupplier.h
@@ -43,10 +43,9 @@ class RDKIT_FILEPARSERS_EXPORT MultithreadedMolSupplier : public MolSupplier {
 
   MultithreadedMolSupplier() {}
 
-  
   // Derived classes MUST have a destructor that calls close
   //  to properly end threads while the instance is alive
-  virtual ~MultithreadedMolSupplier() {close();}
+  virtual ~MultithreadedMolSupplier() { close(); }
 
   //! shut down the supplier
   virtual void close() override;
@@ -133,10 +132,13 @@ class RDKIT_FILEPARSERS_EXPORT MultithreadedMolSupplier : public MolSupplier {
   virtual RWMol *processMoleculeRecord(const std::string &record,
                                        unsigned int lineNum) = 0;
 
-  std::mutex d_threadCounterMutex;
-  std::atomic<unsigned int> d_threadCounter{1};  //!< thread counter
-  std::vector<std::thread> d_writerThreads;      //!< vector writer threads
-  std::thread d_readerThread;                    //!< single reader thread
+  std::vector<std::thread> d_writerThreads;  //!< vector writer threads
+  std::thread d_readerThread;                //!< single reader thread
+
+  // both of these will only be incremented in unique threads
+  // (the reader and the master thread), so no need for atomics/mutexes
+  unsigned int d_mols_read{0};
+  unsigned int d_mols_returned{0};
 
  protected:
   std::atomic<bool> df_started = false;
@@ -160,7 +162,6 @@ class RDKIT_FILEPARSERS_EXPORT MultithreadedMolSupplier : public MolSupplier {
       writeCallback = nullptr;
   std::function<std::string(const std::string &, unsigned int)> readCallback =
       nullptr;
-
 };
 }  // namespace FileParsers
 }  // namespace v2
diff --git a/Code/GraphMol/Wrap/testMultithreadedMolSupplier.py b/Code/GraphMol/Wrap/testMultithreadedMolSupplier.py
index cb72dd37c..46689b930 100644
--- a/Code/GraphMol/Wrap/testMultithreadedMolSupplier.py
+++ b/Code/GraphMol/Wrap/testMultithreadedMolSupplier.py
@@ -168,6 +168,23 @@ class TestCase(unittest.TestCase):
 
     self.assertRaises(ValueError, helper, sdSup)
 
+  def testGitHubIssue8644(self):
+    fileN = os.path.join(RDConfig.RDBaseDir, 'Code', 'GraphMol', 'FileParsers', 'test_data',
+                         'CH.mol')
+    noneMols = 0
+    notNoneMols = 0
+    # this is usually enough to hit the async issue described in the PR
+    # without the test taking forever
+    numIters = 100
+    for i in range(numIters):
+      for m in Chem.MultithreadedSDMolSupplier(fileN, numWriterThreads=100):
+        if m is None:
+          noneMols += 1
+        else:
+          notNoneMols += 1
+    self.assertEqual(notNoneMols, numIters)
+    self.assertEqual(noneMols, 0)
+
 
 if __name__ == '__main__':
   unittest.main()