diff --git a/babel/messages/pofile.py b/babel/messages/pofile.py
index 27f113ddf..f72ddd9d7 100644
--- a/babel/messages/pofile.py
+++ b/babel/messages/pofile.py
@@ -276,7 +276,15 @@ def _process_keyword_line(self, lineno, line, obsolete=False) -> None:
             self.in_msgid = False
             self.in_msgstr = True
             kwarg, has_bracket, idxarg = keyword.partition('[')
-            idx = int(idxarg[:-1]) if has_bracket else 0
+            try:
+                idx = int(idxarg[:-1]) if has_bracket else 0
+            except ValueError:
+                # Nothing was appended to `self.translations`, so leave `in_msgstr` off: a
+                # continuation line after this one would otherwise index an empty list.
+                # This puts the parser in the same state the unknown-keyword path below does.
+                self.in_msgstr = False
+                self._invalid_pofile(line, lineno, f"Invalid plural index in keyword {keyword!r}")
+                return
             s = _NormalizedString(arg) if arg != '""' else _NormalizedString()
             self.translations.append([idx, s])
             return
diff --git a/tests/messages/test_pofile.py b/tests/messages/test_pofile.py
index 0f4b483cf..dedad0c8c 100644
--- a/tests/messages/test_pofile.py
+++ b/tests/messages/test_pofile.py
@@ -177,3 +177,30 @@ def test_issue_1134(case: str, abort_invalid: bool):
         output = pofile.read_po(buf)
         assert len(output) == 1
         assert output["foo"].string in ((''), ('', ''))
+
+
+@pytest.mark.parametrize("abort_invalid", [False, True])
+def test_invalid_msgstr_index_issue_1209(abort_invalid: bool):
+    # Regression test for #1209: a non-integer plural index in msgstr[...] must be reported
+    # through the normal invalid-pofile handling, not leak a bare ValueError from int().
+    buf = StringIO('msgstr[\x0c]')
+
+    if abort_invalid:
+        with pytest.raises(pofile.PoFileError):
+            pofile.read_po(buf, abort_invalid=True)
+    else:
+        # No crash: an invalid entry is skipped with a warning.
+        pofile.read_po(buf)
+
+
+@pytest.mark.parametrize("abort_invalid", [False, True])
+def test_invalid_msgstr_index_continuation_issue_1209(abort_invalid: bool):
+    # The skipped keyword must not leave the parser inside a msgstr: a continuation line
+    # after it would index an empty translations list.
+    buf = StringIO('msgid "foo"\nmsgstr[\x0c] "x"\n"more"\n')
+
+    if abort_invalid:
+        with pytest.raises(pofile.PoFileError):
+            pofile.read_po(buf, abort_invalid=True)
+    else:
+        pofile.read_po(buf)