diff --git a/cms/grading/steps/whitediff.py b/cms/grading/steps/whitediff.py
index f5b7d39e13..f887543578 100644
--- a/cms/grading/steps/whitediff.py
+++ b/cms/grading/steps/whitediff.py
@@ -120,8 +120,12 @@ def _white_diff(output: typing.BinaryIO, res: typing.BinaryIO) -> tuple[bool, st
                     lout = lout[:LENGTH_LIMIT] + b"..."
                 if len(lres) > LENGTH_LIMIT:
                     lres = lres[:LENGTH_LIMIT] + b"..."
-                lout = lout.decode("utf-8", errors='backslashreplace')
-                lres = lres.decode("utf-8", errors='backslashreplace')
+                # NUL is valid UTF-8, so backslashreplace lets it through,
+                # but PostgreSQL cannot store it in a text column.
+                lout = lout.decode("utf-8", errors='backslashreplace') \
+                    .replace("\x00", "\\x00")
+                lres = lres.decode("utf-8", errors='backslashreplace') \
+                    .replace("\x00", "\\x00")
                 return False, f"Expected `{lres}`, found `{lout}` on line {line}"
 
 
diff --git a/cmstestsuite/unit_tests/grading/steps/whitediff_test.py b/cmstestsuite/unit_tests/grading/steps/whitediff_test.py
index 22c54e6e97..66c566e425 100755
--- a/cmstestsuite/unit_tests/grading/steps/whitediff_test.py
+++ b/cmstestsuite/unit_tests/grading/steps/whitediff_test.py
@@ -72,6 +72,13 @@ def test_diff_wrong_long_line(self):
         line2 = line1 + "0"
         self.assertFalse(self._diff(line1, line2))
 
+    def test_diff_nul_byte_escaped_in_admin_text(self):
+        correct, admin_text = _white_diff(
+            BytesIO(b"hello\x00world"), BytesIO(b"helloworld"))
+        self.assertFalse(correct)
+        self.assertNotIn("\x00", admin_text)
+        self.assertIn("hello\\x00world", admin_text)
+
 
 if __name__ == "__main__":
     unittest.main()