diff --git a/thorlcr/thorutil/thbuf.cpp b/thorlcr/thorutil/thbuf.cpp
index ad2d42bdb60..de3093e6a4e 100644
--- a/thorlcr/thorutil/thbuf.cpp
+++ b/thorlcr/thorutil/thbuf.cpp
@@ -1272,6 +1272,10 @@ class CRowSet : public CSimpleInterface, implements IInterface
     {
         return rows.get(r);
     }
+    inline bool isFull() const
+    {
+        return rows.isFull();
+    }
 };
 
 class Chunk : public CInterface
@@ -1395,6 +1399,7 @@ class CSharedWriteAheadBase : public CSimpleInterface, implements ISharedSmartBu
         Owned<CRowSet> outputOwnedRows;
         CRowSet *rowSet;
         unsigned row, rowsInRowSet;
+        bool lastWasNull=false; // for sanity check only (there should never be two consequetive nulls)
 
         void init()
         {
@@ -1464,6 +1469,15 @@ class CSharedWriteAheadBase : public CSimpleInterface, implements ISharedSmartBu
             }
             rowsRead++;
             const void *retrow = rowSet->getRow(row++);
+            if (lastWasNull)
+            {
+                if (retrow)
+                    lastWasNull = false;
+                else
+                    throw makeStringExceptionV(0, "Double null detected - output=%u, rowsRead = %" RCPF "u, rowsWritten = %" RCPF "u, writeAtEof = %s", output, rowsRead, parent.rowsWritten, boolToStr(parent.writeAtEof));
+            }
+            else
+                lastWasNull = (retrow == nullptr);
             return retrow;
         }
         virtual void stop()
@@ -1693,7 +1707,8 @@ class CSharedWriteAheadBase : public CSimpleInterface, implements ISharedSmartBu
         unsigned len=rowSize(row);
         CriticalBlock b(crit);
         bool paged = false;
-        if (totalOutChunkSize >= minChunkSize) // chunks required to be at least minChunkSize
+        // NB: The isFull condition ensures that we never expand inMemRows, which would cause a race with readers reading same set
+        if (totalOutChunkSize >= minChunkSize || inMemRows->isFull()) // chunks required to be at least minChunkSize, or if hits max capacity
         {
             unsigned reader=anyReaderBehind();
             if (NotFound != reader)
diff --git a/thorlcr/thorutil/thmem.hpp b/thorlcr/thorutil/thmem.hpp
index a89d4cdd0f2..f642481f49b 100644
--- a/thorlcr/thorutil/thmem.hpp
+++ b/thorlcr/thorutil/thmem.hpp
@@ -327,7 +327,8 @@ class graph_decl CThorExpandingRowArray : public CSimpleInterface
             if (!resize(numRows+1))
                 return false;
         }
-        rows[numRows++] = row;
+        rows[numRows] = row;
+        numRows++;
         return true;
     }
     bool binaryInsert(const void *row, ICompare &compare, bool dropLast=false); // NB: takes ownership on success
@@ -356,6 +357,7 @@ class graph_decl CThorExpandingRowArray : public CSimpleInterface
     }
     inline rowidx_t ordinality() const { return numRows; }
     inline rowidx_t queryMaxRows() const { return maxRows; }
+    inline bool isFull() const { return numRows >= maxRows; }
 
     inline const void **getRowArray() { return rows; }
     void swap(CThorExpandingRowArray &src);