diff --git a/src/oidcendpoint/oidc/add_on/pkce.py b/src/oidcendpoint/oidc/add_on/pkce.py
index 07bc9f7..f51ffb4 100644
--- a/src/oidcendpoint/oidc/add_on/pkce.py
+++ b/src/oidcendpoint/oidc/add_on/pkce.py
@@ -47,8 +47,11 @@ def post_authn_parse(request, client_id, endpoint_context, **kwargs):
         request["code_challenge_method"] = "plain"
 
     if (
-        request["code_challenge_method"]
-        not in endpoint_context.args["pkce"]["code_challenge_methods"]
+        "code_challenge" in request
+        and (
+            request["code_challenge_method"]
+            not in endpoint_context.args["pkce"]["code_challenge_methods"]
+        )
     ):
         return AuthorizationErrorResponse(
             error="invalid_request",
@@ -119,7 +122,21 @@ def post_token_parse(request, client_id, endpoint_context, **kwargs):
 
 
 def add_pkce_support(endpoint, **kwargs):
-    endpoint["authorization"].post_parse_request.append(post_authn_parse)
+    authn_endpoint = endpoint.get("authorization")
+    if authn_endpoint is None:
+        LOGGER.warning(
+            "No authorization endpoint found, skipping PKCE configuration"
+        )
+        return
+
+    token_endpoint = endpoint.get("token")
+    if token_endpoint is None:
+        LOGGER.warning(
+            "No token endpoint found, skipping PKCE configuration"
+        )
+        return
+
+    authn_endpoint.post_parse_request.append(post_authn_parse)
 
     if "essential" not in kwargs:
         kwargs["essential"] = False
@@ -134,6 +151,6 @@ def add_pkce_support(endpoint, **kwargs):
             raise ValueError("Unsupported method: {}".format(method))
         kwargs["code_challenge_methods"][method] = CC_METHOD[method]
 
-    endpoint["authorization"].endpoint_context.args["pkce"] = kwargs
+    authn_endpoint.endpoint_context.args["pkce"] = kwargs
 
-    endpoint["token"].post_parse_request.append(post_token_parse)
+    token_endpoint.post_parse_request.append(post_token_parse)
diff --git a/tests/test_33_pkce.py b/tests/test_33_pkce.py
index b407520..d11dc65 100644
--- a/tests/test_33_pkce.py
+++ b/tests/test_33_pkce.py
@@ -371,3 +371,54 @@ def test_no_code_verifier(self):
         assert isinstance(resp, TokenErrorResponse)
         assert resp["error"] == "invalid_grant"
         assert resp["error_description"] == "Missing code_verifier"
+
+    def test_no_authorization_endpoint(self, conf, caplog):
+        """
+        Test that PKCE configuration does not crash when there is no authorization
+        endpoint and a warning is logged.
+        """
+        del conf["endpoint"]["authorization"]
+        create_endpoint(conf)
+        assert "WARNING" in caplog.text
+        assert (
+            "No authorization endpoint found, skipping PKCE configuration"
+            in caplog.text
+        )
+
+    def test_no_token_endpoint(self, conf, caplog):
+        """
+        Test that PKCE configuration does not crash when there is no token endpoint
+        and a warning is logged.
+        """
+        del conf["endpoint"]["token"]
+        create_endpoint(conf)
+        assert "WARNING" in caplog.text
+        assert "No token endpoint found, skipping PKCE configuration" in caplog.text
+
+    def test_plain_challenge_method_not_supported_and_PKCE_not_essential(self, conf):
+        """
+        Test that an authentication request without PKCE parameters does not fail when
+        "plain" code_challenge_method is not supported and PKCE is not essential.
+        """
+        conf["add_on"]["pkce"]["kwargs"]["code_challenge_methods"] = ["S256"]
+        conf["add_on"]["pkce"]["kwargs"]["essential"] = False
+        endpoint_context = create_endpoint(conf)
+        authn_endpoint = endpoint_context.endpoint["authorization"]
+        token_endpoint = endpoint_context.endpoint["token"]
+
+        authentication_request = AUTH_REQ.copy()
+
+        parsed_request = authn_endpoint.parse_request(authentication_request.to_dict())
+
+        assert not isinstance(parsed_request, AuthorizationErrorResponse)
+        assert isinstance(parsed_request, AuthorizationRequest)
+
+        response = authn_endpoint.process_request(parsed_request)
+
+        assert isinstance(response["response_args"], AuthorizationResponse)
+
+        token_request = TOKEN_REQ.copy()
+        token_request["code"] = response["response_args"]["code"]
+        parsed_token_request = token_endpoint.parse_request(token_request)
+
+        assert isinstance(parsed_token_request, AccessTokenRequest)