diff --git a/packaging/_parser.py b/packaging/_parser.py
index 103f679a..d03119a1 100644
--- a/packaging/_parser.py
+++ b/packaging/_parser.py
@@ -327,5 +327,5 @@ def _parse_marker_op(tokenizer: Tokenizer) -> Op:
     else:
         return tokenizer.raise_syntax_error(
             "Expected marker operator, one of "
-            "<=, <, !=, ==, >=, >, ~=, ===, not, not in"
+            "<=, <, !=, ==, >=, >, ~=, ===, in, not in"
         )
diff --git a/tests/test_requirements.py b/tests/test_requirements.py
index 93839d69..92cf9ce6 100644
--- a/tests/test_requirements.py
+++ b/tests/test_requirements.py
@@ -391,7 +391,7 @@ def test_error_invalid_marker_notin_without_whitespace(self) -> None:
         assert ctx.exconly() == (
             "packaging.requirements.InvalidRequirement: "
             "Expected marker operator, one of <=, <, !=, ==, >=, >, ~=, ===, "
-            "not, not in\n"
+            "in, not in\n"
             "    name; '3.7' notin python_version\n"
             "                ^"
         )
@@ -408,7 +408,7 @@ def test_error_when_no_word_boundary(self) -> None:
         assert ctx.exconly() == (
             "packaging.requirements.InvalidRequirement: "
             "Expected marker operator, one of <=, <, !=, ==, >=, >, ~=, ===, "
-            "not, not in\n"
+            "in, not in\n"
             "    name; '3.6'inpython_version\n"
             "               ^"
         )
@@ -441,7 +441,7 @@ def test_error_invalid_marker_with_invalid_op(self) -> None:
         assert ctx.exconly() == (
             "packaging.requirements.InvalidRequirement: "
             "Expected marker operator, one of <=, <, !=, ==, >=, >, ~=, ===, "
-            "not, not in\n"
+            "in, not in\n"
             "    name; '3.7' ~ python_version\n"
             "                ^"
         )