diff --git a/server/src/server/router/api/v1/users/_userID/games/_game/_playtype/settings/router.test.ts b/server/src/server/router/api/v1/users/_userID/games/_game/_playtype/settings/router.test.ts
index 19c14ed5b..49e994f48 100644
--- a/server/src/server/router/api/v1/users/_userID/games/_game/_playtype/settings/router.test.ts
+++ b/server/src/server/router/api/v1/users/_userID/games/_game/_playtype/settings/router.test.ts
@@ -136,6 +136,33 @@ t.test("PATCH /api/v1/users/:userID/games/:game/:playtype/settings", (t) => {
 		t.end();
 	});
 
+	t.test("Should reject a arbitrary things on game specific settings.", async (t) => {
+		await db["api-tokens"].insert({
+			userID: 1,
+			identifier: "api_token",
+			permissions: {
+				customise_profile: true,
+			},
+			token: "api_token",
+			fromAPIClient: null,
+		});
+
+		const res = await mockApi
+			.patch("/api/v1/users/1/games/iidx/SP/settings")
+			.set("Authorization", "Bearer api_token")
+			.send({
+				preferredScoreAlg: "ktRating",
+				gameSpecific: {
+					display2DXTra: true,
+					arbitraryValue: {},
+				},
+			});
+
+		t.equal(res.statusCode, 400);
+
+		t.end();
+	});
+
 	t.test("Requires the user to be authed as the requested user.", async (t) => {
 		await db["api-tokens"].insert({
 			userID: 2,
diff --git a/server/src/server/router/api/v1/users/_userID/games/_game/_playtype/settings/router.ts b/server/src/server/router/api/v1/users/_userID/games/_game/_playtype/settings/router.ts
index ed865c379..e78e707fe 100644
--- a/server/src/server/router/api/v1/users/_userID/games/_game/_playtype/settings/router.ts
+++ b/server/src/server/router/api/v1/users/_userID/games/_game/_playtype/settings/router.ts
@@ -55,12 +55,10 @@ router.patch(
 					display2DXTra: "boolean",
 				};
 			}
-			const err = p(
-				req.body,
-				{ gameSpecific: p.optional(schema) },
-				{},
-				{ allowExcessKeys: true }
-			);
+			// A limitation in prudence means that validating top-level properties like this isn't
+			// possible.
+			// A prudence v1.0. should fix this!
+			const err = p({ __: req.body.gameSpecific }, { __: p.optional(schema) }, {});
 
 			if (err) {
 				return res.status(400).json({