diff --git a/etc/eslint.cjs b/etc/eslint.cjs
index d4aca928..f81ea9f8 100644
--- a/etc/eslint.cjs
+++ b/etc/eslint.cjs
@@ -6,15 +6,19 @@ module.exports = {
 	},
 	extends: [
 		"eslint:recommended",
-		"plugin:@typescript-eslint/recommended"
+		"plugin:@typescript-eslint/recommended-type-checked"
 	],
 	overrides: [
 		{
-			files: ["*.cjs", "*.js", "*.mjs"],
+			files: ["*.{cjs,js,mjs}"],
 			rules: {
 				"@typescript-eslint/explicit-function-return-type": "off",
 				"@typescript-eslint/explicit-module-boundary-types": "off"
 			}
+		},
+		{
+			files: ["test/**/*.js"],
+			rules: {"@typescript-eslint/no-floating-promises": "off"}
 		}
 	],
 	parser: "@typescript-eslint/parser",
@@ -135,7 +139,7 @@ module.exports = {
 		"no-useless-rename": "error",
 		"no-useless-return": "error",
 		"no-var": "error",
-		"no-void": "error",
+		"no-void": ["error", {allowAsStatement: true}],
 		"no-warning-comments": "warn",
 		"object-shorthand": "error",
 		"one-var": ["error", "never"],
@@ -190,7 +194,7 @@ module.exports = {
 		"@typescript-eslint/naming-convention": "off",
 		"@typescript-eslint/no-array-delete": "error",
 		"@typescript-eslint/no-confusing-non-null-assertion": "error",
-		"@typescript-eslint/no-confusing-void-expression": ["error", {ignoreArrowShorthand: true}],
+		"@typescript-eslint/no-confusing-void-expression": "off",
 		"@typescript-eslint/no-dupe-class-members": "error",
 		"@typescript-eslint/no-dynamic-delete": "error",
 		"@typescript-eslint/no-empty-function": "error",
@@ -243,6 +247,7 @@ module.exports = {
 		"@typescript-eslint/prefer-ts-expect-error": "error",
 		"@typescript-eslint/promise-function-async": "off",
 		"@typescript-eslint/require-array-sort-compare": "error",
+		"@typescript-eslint/restrict-template-expressions": "off",
 		"@typescript-eslint/return-await": "error",
 		"@typescript-eslint/sort-type-constituents": "error",
 		"@typescript-eslint/strict-boolean-expressions": "off",
diff --git a/example/check_comment.js b/example/check_comment.js
index 59e7544d..b2310ce3 100644
--- a/example/check_comment.js
+++ b/example/check_comment.js
@@ -29,5 +29,6 @@ try {
 	console.log(result == CheckResult.ham ? "The comment is ham." : "The comment is spam.");
 }
 catch (error) {
-	console.log(`An error occurred: ${error}`);
+	const message = error instanceof Error ? error.message : String(error);
+	console.log(`An error occurred: ${message}`);
 }
diff --git a/example/submit_ham.js b/example/submit_ham.js
index ae66ea93..3062cbcb 100644
--- a/example/submit_ham.js
+++ b/example/submit_ham.js
@@ -18,5 +18,6 @@ try {
 	console.log("The comment was successfully submitted as ham.");
 }
 catch (error) {
-	console.log(`An error occurred: ${error}`);
+	const message = error instanceof Error ? error.message : String(error);
+	console.log(`An error occurred: ${message}`);
 }
diff --git a/example/submit_spam.js b/example/submit_spam.js
index cf103f63..5e6abb61 100644
--- a/example/submit_spam.js
+++ b/example/submit_spam.js
@@ -18,5 +18,6 @@ try {
 	console.log("The comment was successfully submitted as spam.");
 }
 catch (error) {
-	console.log(`An error occurred: ${error}`);
+	const message = error instanceof Error ? error.message : String(error);
+	console.log(`An error occurred: ${message}`);
 }
diff --git a/example/verify_key.js b/example/verify_key.js
index 581ce227..64825c01 100644
--- a/example/verify_key.js
+++ b/example/verify_key.js
@@ -10,5 +10,6 @@ try {
 	console.log(isValid ? "The API key is valid." : "The API key is invalid.");
 }
 catch (error) {
-	console.log(`An error occurred: ${error}`);
+	const message = error instanceof Error ? error.message : String(error);
+	console.log(`An error occurred: ${message}`);
 }
diff --git a/package-lock.json b/package-lock.json
index 7b37ed86..ecbc2913 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -9,7 +9,7 @@
 			"version": "16.1.0",
 			"license": "MIT",
 			"devDependencies": {
-				"@types/node": "^20.11.9",
+				"@types/node": "^20.11.10",
 				"@typescript-eslint/eslint-plugin": "^6.19.1",
 				"@typescript-eslint/parser": "^6.19.1",
 				"del": "^7.1.0",
@@ -206,9 +206,9 @@
 			"dev": true
 		},
 		"node_modules/@types/node": {
-			"version": "20.11.9",
-			"resolved": "https://registry.npmjs.org/@types/node/-/node-20.11.9.tgz",
-			"integrity": "sha512-CQXNuMoS/VcoAMISe5pm4JnEd1Br5jildbQEToEMQvutmv+EaQr90ry9raiudgpyDuqFiV9e4rnjSfLNq12M5w==",
+			"version": "20.11.10",
+			"resolved": "https://registry.npmjs.org/@types/node/-/node-20.11.10.tgz",
+			"integrity": "sha512-rZEfe/hJSGYmdfX9tvcPMYeYPW2sNl50nsw4jZmRcaG0HIAb0WYEpsB05GOb53vjqpyE9GUhlDQ4jLSoB5q9kg==",
 			"dev": true,
 			"dependencies": {
 				"undici-types": "~5.26.4"
diff --git a/package.json b/package.json
index b34826dc..a6054aaa 100644
--- a/package.json
+++ b/package.json
@@ -13,7 +13,7 @@
 		"url": "https://belin.io"
 	},
 	"devDependencies": {
-		"@types/node": "^20.11.9",
+		"@types/node": "^20.11.10",
 		"@typescript-eslint/eslint-plugin": "^6.19.1",
 		"@typescript-eslint/parser": "^6.19.1",
 		"del": "^7.1.0",
@@ -50,8 +50,8 @@
 	"scripts": {
 		"build": "node tool/build.js && tsc --project src/tsconfig.json",
 		"clean": "node tool/clean.js",
-		"doc": "typedoc --options etc/typedoc.js && node tool/doc.js",
 		"dist": "npm run clean && npm run build",
+		"doc": "typedoc --options etc/typedoc.js && node tool/doc.js",
 		"lint": "tsc --project tsconfig.json && eslint --config=etc/eslint.cjs example src test tool",
 		"postpublish": "node tool/publish.js",
 		"prepack": "npm run dist",
diff --git a/src/client.ts b/src/client.ts
index e0c726bd..3f6eee72 100644
--- a/src/client.ts
+++ b/src/client.ts
@@ -105,7 +105,7 @@ export class Client {
 	 * @param fields The fields describing the query body.
 	 * @returns The server response.
 	 */
-	async #fetch(endpoint: string, fields: Record<string, string>): Promise<Response> {
+	async #fetch(endpoint: string, fields: Record<string, string[]|string>): Promise<Response> {
 		const body = new URLSearchParams(this.blog.toJSON());
 		body.set("api_key", this.apiKey);
 		if (this.isTest) body.set("is_test", "1");
diff --git a/src/comment.ts b/src/comment.ts
index d363e324..3d8d73ef 100644
--- a/src/comment.ts
+++ b/src/comment.ts
@@ -90,7 +90,7 @@ export class Comment {
 	 * Converts this object to a map in JSON format.
 	 * @returns The map in JSON format corresponding to this object.
 	 */
-	toJSON(): Record<string, any> {
+	toJSON(): Record<string, string[]|string> {
 		const map = this.author ? this.author.toJSON() : {};
 		if (this.content) map.comment_content = this.content;
 		if (this.context.length) map.comment_context = this.context;
diff --git a/src/usage.ts b/src/usage.ts
index b3c1b236..abc33bcd 100644
--- a/src/usage.ts
+++ b/src/usage.ts
@@ -41,10 +41,10 @@ export class Usage {
 	 */
 	static fromJson(json: Record<string, any>): Usage {
 		return new this({
-			limit: Number.isInteger(json.limit) ? json.limit : -1,
+			limit: typeof json.limit == "number" && Number.isInteger(json.limit) ? json.limit : -1,
 			percentage: typeof json.percentage == "number" ? json.percentage : 0,
 			throttled: typeof json.throttled == "boolean" ? json.throttled : false,
-			usage: Number.isInteger(json.usage) ? json.usage : 0
+			usage: typeof json.usage == "number" && Number.isInteger(json.usage) ? json.usage : 0
 		});
 	}
 }