--- a/src/main/java/tech/streamfusion/arrow/ArrowConversion.java
+++ b/src/main/java/tech/streamfusion/arrow/ArrowConversion.java
@@ -254,7 +254,15 @@
     List<FieldVector> vectors = root.getFieldVectors();
     for (int i = 0; i < vectors.size(); i++) {
       FieldVector vector = vectors.get(i);
-      if (fixedWidthCapacity >= 0 && vector instanceof org.apache.arrow.vector.BaseFixedWidthVector fixed) {
+      if (fixedWidthCapacity > 0 && vector instanceof FixedSizeBinaryVector fixed) {
+        try (var validity = fixed.getAllocator().buffer((fixedWidthCapacity + 7L) / 8);
+             var values = fixed.getAllocator().buffer((long) fixedWidthCapacity * fixed.getByteWidth())) {
+          validity.setZero(0, validity.capacity());
+          fixed.loadFieldBuffers(
+              new org.apache.arrow.vector.ipc.message.ArrowFieldNode(0, 0),
+              Arrays.asList(validity, values));
+        }
+      } else if (fixedWidthCapacity >= 0 && vector instanceof org.apache.arrow.vector.BaseFixedWidthVector fixed) {
         fixed.allocateNew(fixedWidthCapacity);
       } else {
         vector.allocateNew();
--- a/src/main/java/tech/streamfusion/arrow/writers/BinaryWriter.java
+++ b/src/main/java/tech/streamfusion/arrow/writers/BinaryWriter.java
@@ -49,7 +49,10 @@
     @Override
     public void doWrite(T in, int ordinal) {
         if (isNullAt(in, ordinal)) {
-            ((FixedSizeBinaryVector) getValueVector()).setNull(getCount());
+            var vector = (FixedSizeBinaryVector) getValueVector();
+            vector.setNull(getCount());
+            vector.getDataBuffer().setZero(
+                    (long) getCount() * vector.getByteWidth(), vector.getByteWidth());
         } else {
             ((FixedSizeBinaryVector) getValueVector()).setSafe(getCount(), readBinary(in, ordinal));
         }