diff --git a/src/ass/line.py b/src/ass/line.py
index 5d1defc..5dbbc98 100644
--- a/src/ass/line.py
+++ b/src/ass/line.py
@@ -41,7 +41,11 @@ def dump(self, field_order=None):
         if field_order is None:
             field_order = self.DEFAULT_FIELD_ORDER
 
-        return ",".join(_Field.dump(self.fields[field])
+        # A field in field_order that the line does not carry (for example when
+        # a section has a second "Format:" line that redefines the order after
+        # the lines were parsed) dumps as empty rather than raising KeyError.
+        return ",".join(_Field.dump(self.fields[field]) if field in self.fields
+                        else ""
                         for field in field_order)
 
     def dump_with_type(self, field_order=None):
diff --git a/tests/test_ass.py b/tests/test_ass.py
index 317880c..a3586bb 100755
--- a/tests/test_ass.py
+++ b/tests/test_ass.py
@@ -135,6 +135,19 @@ def test_custom_line_section_write(self, line_section):
     def test_custom_line_section_dump(self, line_section):
         assert "\n".join(line_section.dump()) == self.TEST_CUSTOM
 
+    def test_dump_after_format_redefinition(self):
+        # A second "Format:" line in a section redefines the field order after
+        # the lines were parsed; dumping such a document used to raise KeyError.
+        doc = ass.Document.parse_string(dedent("""\
+            [Events]
+            Format: Layer, Start, End, Style, Text
+            Dialogue: 0,0:00:01.00,0:00:04.00,Default,Hello
+            Format: a"""))
+        out = StringIO()
+        doc.dump_file(out)
+        # the dumped document parses again without error
+        assert ass.Document.parse_string(out.getvalue()) is not None
+
 
 class TestEvents: