diff --git a/core/src/main/java/google/registry/export/sheet/SheetSynchronizer.java b/core/src/main/java/google/registry/export/sheet/SheetSynchronizer.java
index 8b6ec15b1..c1a46eca7 100644
--- a/core/src/main/java/google/registry/export/sheet/SheetSynchronizer.java
+++ b/core/src/main/java/google/registry/export/sheet/SheetSynchronizer.java
@@ -38,6 +38,8 @@ class SheetSynchronizer {
 
   private static final String SHEET_NAME = "Registrars";
 
+  private static final String INVALID_HEADER_TEXT = "Invalid Sheet column header name";
+
   @Inject Sheets sheetsService;
   @Inject SheetSynchronizer() {}
 
@@ -92,10 +94,12 @@ class SheetSynchronizer {
         cellRow.add("");
       }
       for (int j = 0; j < headers.size(); j++) {
-        // Look for the value corresponding to the row and header indices in data
-        String dataField = data.get(i).get(headers.get(j));
-        // If the cell's header matches a data header, and the values aren't equal, mutate it
-        if (dataField != null && !cellRow.get(j).toString().equals(dataField)) {
+        // Look for the value corresponding to the row and header indices in data, otherwise write
+        // out an error so it's clear to a reader of the Google Sheet that the column header is
+        // invalid.
+        String dataField = data.get(i).getOrDefault(headers.get(j), INVALID_HEADER_TEXT);
+        // If the existing cell value is different than the new data, mutate it.
+        if (!cellRow.get(j).toString().equals(dataField)) {
           mutated = true;
           originalVals.get(i).set(j, dataField);
         }
diff --git a/core/src/test/java/google/registry/export/sheet/SheetSynchronizerTest.java b/core/src/test/java/google/registry/export/sheet/SheetSynchronizerTest.java
index 7b20c6e0b..16a05a4e8 100644
--- a/core/src/test/java/google/registry/export/sheet/SheetSynchronizerTest.java
+++ b/core/src/test/java/google/registry/export/sheet/SheetSynchronizerTest.java
@@ -114,7 +114,7 @@ class SheetSynchronizerTest {
   }
 
   @Test
-  void testSynchronize_unknownFields_doesntUpdate() throws Exception {
+  void testSynchronize_unknownFields_writesColumnHeaderErrorIntoCell() throws Exception {
     existingSheet.add(createRow("a", "c", "b"));
     existingSheet.add(createRow("diffVal1", "sameVal", "diffVal2"));
     data = ImmutableList.of(ImmutableMap.of("a", "val1", "b", "val2", "d", "val3"));
@@ -125,7 +125,7 @@ class SheetSynchronizerTest {
 
     BatchUpdateValuesRequest expectedRequest = new BatchUpdateValuesRequest();
     List<List<Object>> expectedVals = newArrayList();
-    expectedVals.add(createRow("val1", "sameVal", "val2"));
+    expectedVals.add(createRow("val1", "Invalid Sheet column header name", "val2"));
     expectedRequest.setData(
         newArrayList(new ValueRange().setRange("Registrars!A2").setValues(expectedVals)));
     expectedRequest.setValueInputOption("RAW");
@@ -133,7 +133,7 @@ class SheetSynchronizerTest {
   }
 
   @Test
-  void testSynchronize_notFullRow_getsPadded() throws Exception {
+  void testSynchronize_notFullRow_isHandledCorrectly() throws Exception {
     existingSheet.add(createRow("a", "c", "b"));
     existingSheet.add(createRow("diffVal1", "diffVal2"));
     data = ImmutableList.of(ImmutableMap.of("a", "val1", "b", "paddedVal", "d", "val3"));
@@ -144,7 +144,7 @@ class SheetSynchronizerTest {
 
     BatchUpdateValuesRequest expectedRequest = new BatchUpdateValuesRequest();
     List<List<Object>> expectedVals = newArrayList();
-    expectedVals.add(createRow("val1", "diffVal2", "paddedVal"));
+    expectedVals.add(createRow("val1", "Invalid Sheet column header name", "paddedVal"));
     expectedRequest.setData(
         newArrayList(new ValueRange().setRange("Registrars!A2").setValues(expectedVals)));
     expectedRequest.setValueInputOption("RAW");