diff --git a/package.json b/package.json
index 14e3273..f079161 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "@samchon/openapi",
-  "version": "0.1.19",
+  "version": "0.1.20",
   "description": "OpenAPI definitions and converters for 'typia' and 'nestia'.",
   "main": "./lib/index.js",
   "typings": "./lib/index.d.ts",
@@ -29,6 +29,7 @@
   },
   "homepage": "https://github.com/samchon/openapi",
   "devDependencies": {
+    "@nestia/e2e": "^0.4.3",
     "@types/node": "^20.12.7",
     "prettier": "^3.2.5",
     "rimraf": "^5.0.5",
diff --git a/src/internal/OpenApiV3Downgrader.ts b/src/internal/OpenApiV3Downgrader.ts
index 9501c6a..c273207 100644
--- a/src/internal/OpenApiV3Downgrader.ts
+++ b/src/internal/OpenApiV3Downgrader.ts
@@ -258,26 +258,27 @@ export namespace OpenApiV3Downgrader {
         const insert = (value: any): void => {
           const matched: OpenApiV3.IJsonSchema.INumber | undefined = union.find(
             (u) =>
-              (u as OpenApiV3.IJsonSchema.__ISignificant<any>).type === value,
+              (u as OpenApiV3.IJsonSchema.__ISignificant<any>).type ===
+              typeof value,
           ) as OpenApiV3.IJsonSchema.INumber | undefined;
           if (matched !== undefined) {
             matched.enum ??= [];
             matched.enum.push(value);
           } else union.push({ type: typeof value as "number", enum: [value] });
-          if (OpenApiTypeChecker.isConstant(schema)) insert(schema.const);
-          else if (OpenApiTypeChecker.isOneOf(schema))
-            schema.oneOf.forEach(insert);
         };
+        if (OpenApiTypeChecker.isConstant(schema)) insert(schema.const);
+        else if (OpenApiTypeChecker.isOneOf(schema))
+          for (const u of schema.oneOf)
+            if (OpenApiTypeChecker.isConstant(u)) insert(u.const);
       };
 
       visit(input);
       visitConstant(input);
-      if (nullable)
+      if (nullable === true)
         for (const u of union)
           if (OpenApiTypeChecker.isReference(u))
             downgradeNullableReference(collection)(u);
           else (u as OpenApiV3.IJsonSchema.IArray).nullable = true;
-
       if (nullable === true && union.length === 0)
         return { type: "null", ...attribute };
       return {
diff --git a/test/features/test_downgrade_v20.ts b/test/features/test_downgrade_v20.ts
new file mode 100644
index 0000000..c3da255
--- /dev/null
+++ b/test/features/test_downgrade_v20.ts
@@ -0,0 +1,23 @@
+import { OpenApi, SwaggerV2 } from "../../src";
+import { TestValidator } from "@nestia/e2e";
+import { SwaggerV2Downgrader } from "../../src/internal/SwaggerV2Downgrader";
+
+export const test_downgrade_v20 = () => {
+  const schema: OpenApi.IJsonSchema = {
+    oneOf: [{ const: "a" }, { const: "b" }, { const: "c" }],
+    title: "something",
+    description: "nothing",
+  };
+  const downgraded: SwaggerV2.IJsonSchema = SwaggerV2Downgrader.downgradeSchema(
+    {
+      original: {},
+      downgraded: {},
+    },
+  )(schema);
+  TestValidator.equals("enum")(downgraded)({
+    type: "string",
+    title: "something",
+    description: "nothing",
+    enum: ["a", "b", "c"],
+  });
+};
diff --git a/test/features/test_downgrade_v30.ts b/test/features/test_downgrade_v30.ts
new file mode 100644
index 0000000..da97c89
--- /dev/null
+++ b/test/features/test_downgrade_v30.ts
@@ -0,0 +1,23 @@
+import { OpenApi, OpenApiV3 } from "../../src";
+import { OpenApiV3Downgrader } from "../../src/internal/OpenApiV3Downgrader";
+import { TestValidator } from "@nestia/e2e";
+
+export const test_downgrade_v30 = () => {
+  const schema: OpenApi.IJsonSchema = {
+    oneOf: [{ const: "a" }, { const: "b" }, { const: "c" }],
+    title: "something",
+    description: "nothing",
+  };
+  const downgraded: OpenApiV3.IJsonSchema = OpenApiV3Downgrader.downgradeSchema(
+    {
+      original: {},
+      downgraded: {},
+    },
+  )(schema);
+  TestValidator.equals("enum")(downgraded)({
+    type: "string",
+    title: "something",
+    description: "nothing",
+    enum: ["a", "b", "c"],
+  });
+};
diff --git a/test/index.ts b/test/index.ts
index 5d42503..08ddbef 100644
--- a/test/index.ts
+++ b/test/index.ts
@@ -3,6 +3,8 @@ import path from "path";
 import typia from "typia";
 
 import { OpenApi, OpenApiV3, OpenApiV3_1, SwaggerV2 } from "../src";
+import { test_downgrade_v30 } from "./features/test_downgrade_v30";
+import { test_downgrade_v20 } from "./features/test_downgrade_v20";
 
 const CONVERTED: string = `${__dirname}/../../examples/converted`;
 
@@ -68,6 +70,9 @@ const main = async (): Promise<void> => {
   await iterate(`${__dirname}/../../examples/v2.0`);
   await iterate(`${__dirname}/../../examples/v3.0`);
   await iterate(`${__dirname}/../../examples/v3.1`);
+
+  test_downgrade_v20();
+  test_downgrade_v30();
 };
 main().catch((exp) => {
   console.error(exp);