diff --git a/processing/src/main/java/edu/harvard/hms/dbmi/avillach/hpds/processing/io/PfbWriter.java b/processing/src/main/java/edu/harvard/hms/dbmi/avillach/hpds/processing/io/PfbWriter.java
index 31f8c9b7..2d098a7a 100644
--- a/processing/src/main/java/edu/harvard/hms/dbmi/avillach/hpds/processing/io/PfbWriter.java
+++ b/processing/src/main/java/edu/harvard/hms/dbmi/avillach/hpds/processing/io/PfbWriter.java
@@ -52,7 +52,7 @@ public void writeHeader(String[] data) {
         SchemaBuilder.FieldAssembler<Schema> patientRecords = SchemaBuilder.record("patientData")
                 .fields();
 
-        fields.forEach(patientRecords::requiredString);
+        fields.forEach(field -> patientRecords.nullableString(field, "null"));
         patientDataSchema = patientRecords.endRecord();
 
         Schema objectSchema = Schema.createUnion(metadataSchema, patientDataSchema);
diff --git a/processing/src/test/java/edu/harvard/hms/dbmi/avillach/hpds/processing/io/PfbWriterTest.java b/processing/src/test/java/edu/harvard/hms/dbmi/avillach/hpds/processing/io/PfbWriterTest.java
index 9a527838..4094dd93 100644
--- a/processing/src/test/java/edu/harvard/hms/dbmi/avillach/hpds/processing/io/PfbWriterTest.java
+++ b/processing/src/test/java/edu/harvard/hms/dbmi/avillach/hpds/processing/io/PfbWriterTest.java
@@ -19,7 +19,7 @@ public void writeValidPFB() {
         pfbWriter.writeHeader(new String[] {"\\demographics\\age\\", "\\phs123\\stroke\\"});
         pfbWriter.writeEntity(List.of(new String[]{"80", "Y"},
                 new String[]{"70", "N"},
-                new String[]{"75", "N"}
+                new String[]{"75", null}
         ));
         pfbWriter.close();
         // todo: validate this programatically