diff --git a/src/graphql.ts b/src/graphql.ts
index 51416209..58b3b922 100644
--- a/src/graphql.ts
+++ b/src/graphql.ts
@@ -4,6 +4,7 @@ import {
   DefinitionNode,
   DocumentNode,
   GraphQLSchema,
+  isSpecifiedScalarType,
   ListTypeNode,
   NamedTypeNode,
   NameNode,
@@ -169,3 +170,8 @@ export const topsort = (g: Graph): string[] => {
 
   return results.reverse();
 };
+
+export const isGeneratedByIntrospection = (schema: GraphQLSchema): boolean =>
+  Object.entries(schema.getTypeMap())
+    .filter(([name, type]) => !name.startsWith('__') && !isSpecifiedScalarType(type))
+    .every(([, type]) => type.astNode === undefined);
diff --git a/src/index.ts b/src/index.ts
index a970eec1..0f05b117 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -1,9 +1,9 @@
 import { PluginFunction, Types } from '@graphql-codegen/plugin-helpers';
 import { transformSchemaAST } from '@graphql-codegen/schema-ast';
-import { GraphQLSchema, visit } from 'graphql';
+import { buildSchema, GraphQLSchema, printSchema, visit } from 'graphql';
 
 import { ValidationSchemaPluginConfig } from './config';
-import { topologicalSortAST } from './graphql';
+import { isGeneratedByIntrospection, topologicalSortAST } from './graphql';
 import { MyZodSchemaVisitor } from './myzod/index';
 import { SchemaVisitor } from './types';
 import { YupSchemaVisitor } from './yup/index';
@@ -40,16 +40,20 @@ const schemaVisitor = (schema: GraphQLSchema, config: ValidationSchemaPluginConf
 
 const _transformSchemaAST = (schema: GraphQLSchema, config: ValidationSchemaPluginConfig) => {
   const { schema: _schema, ast } = transformSchemaAST(schema, config);
+
+  // See: https://github.com/Code-Hex/graphql-codegen-typescript-validation-schema/issues/394
+  const __schema = isGeneratedByIntrospection(_schema) ? buildSchema(printSchema(_schema)) : _schema;
+
   // This affects the performance of code generation, so it is
   // enabled only when this option is selected.
   if (config.validationSchemaExportType === 'const') {
     return {
-      schema: _schema,
-      ast: topologicalSortAST(_schema, ast),
+      schema: __schema,
+      ast: topologicalSortAST(__schema, ast),
     };
   }
   return {
-    schema: _schema,
+    schema: __schema,
     ast,
   };
 };
diff --git a/tests/graphql.spec.ts b/tests/graphql.spec.ts
index 8fa75572..4e34ebcb 100644
--- a/tests/graphql.spec.ts
+++ b/tests/graphql.spec.ts
@@ -1,8 +1,16 @@
 import { Graph } from 'graphlib';
-import { buildSchema, Kind, ObjectTypeDefinitionNode, parse, print } from 'graphql';
+import {
+  buildClientSchema,
+  buildSchema,
+  introspectionFromSchema,
+  Kind,
+  ObjectTypeDefinitionNode,
+  parse,
+  print,
+} from 'graphql';
 import dedent from 'ts-dedent';
 
-import { ObjectTypeDefinitionBuilder, topologicalSortAST, topsort } from '../src/graphql';
+import { isGeneratedByIntrospection, ObjectTypeDefinitionBuilder, topologicalSortAST, topsort } from '../src/graphql';
 
 describe('graphql', () => {
   describe('ObjectTypeDefinitionBuilder', () => {
@@ -238,3 +246,53 @@ describe('topologicalSortAST', () => {
     expect(sortedSchema).toBe(expectedSortedSchema);
   });
 });
+
+describe('isGeneratedByIntrospection function', () => {
+  const schemaDefinition = /* GraphQL */ `
+    scalar CustomScalar
+
+    interface Node {
+      id: ID!
+    }
+
+    type UserType implements Node {
+      id: ID!
+      name: String!
+      email: String!
+    }
+
+    union SearchResult = UserType
+
+    enum Role {
+      ADMIN
+      USER
+    }
+
+    input UserInput {
+      name: String!
+      email: String!
+      role: Role!
+    }
+
+    type Query {
+      user(id: ID!): UserType!
+      search(text: String!): [SearchResult]
+    }
+
+    type Mutation {
+      createUser(input: UserInput!): UserType!
+    }
+  `;
+
+  test('returns false for a schema not generated by introspection', () => {
+    const schema = buildSchema(schemaDefinition);
+    expect(isGeneratedByIntrospection(schema)).toBe(false);
+  });
+
+  test('returns true for a schema generated by introspection', () => {
+    const schema = buildSchema(schemaDefinition);
+    const query = introspectionFromSchema(schema);
+    const clientSchema = buildClientSchema(query);
+    expect(isGeneratedByIntrospection(clientSchema)).toBe(true);
+  });
+});
diff --git a/tests/myzod.spec.ts b/tests/myzod.spec.ts
index 0bbf7e8a..3c9826d8 100644
--- a/tests/myzod.spec.ts
+++ b/tests/myzod.spec.ts
@@ -1,4 +1,4 @@
-import { buildSchema } from 'graphql';
+import { buildClientSchema, buildSchema, introspectionFromSchema } from 'graphql';
 import dedent from 'ts-dedent';
 
 import { plugin } from '../src/index';
@@ -922,4 +922,41 @@ describe('myzod', () => {
       expect(result.content).not.toContain(wantNotContain);
     }
   });
+
+  it('issue #394', async () => {
+    const schema = buildSchema(/* GraphQL */ `
+      enum Test {
+        A
+        B
+      }
+
+      type Query {
+        _dummy: Test
+      }
+
+      input QueryInput {
+        _dummy: Test
+      }
+    `);
+    const query = introspectionFromSchema(schema);
+    const clientSchema = buildClientSchema(query);
+    const result = await plugin(
+      clientSchema,
+      [],
+      {
+        schema: 'myzod',
+        scalars: {
+          ID: 'string',
+        },
+      },
+      {}
+    );
+    const wantContain = dedent`
+    export function QueryInputSchema(): myzod.Type<QueryInput> {
+      return myzod.object({
+        _dummy: TestSchema.optional().nullable()
+      })
+    }`;
+    expect(result.content).toContain(wantContain);
+  });
 });
diff --git a/tests/yup.spec.ts b/tests/yup.spec.ts
index 86fd10d7..5112f8a2 100644
--- a/tests/yup.spec.ts
+++ b/tests/yup.spec.ts
@@ -1,4 +1,4 @@
-import { buildSchema } from 'graphql';
+import { buildClientSchema, buildSchema, introspectionFromSchema } from 'graphql';
 import dedent from 'ts-dedent';
 
 import { plugin } from '../src/index';
@@ -836,4 +836,41 @@ describe('yup', () => {
       expect(result.content).not.toContain(wantNotContain);
     }
   });
+
+  it('issue #394', async () => {
+    const schema = buildSchema(/* GraphQL */ `
+      enum Test {
+        A
+        B
+      }
+
+      type Query {
+        _dummy: Test
+      }
+
+      input QueryInput {
+        _dummy: Test
+      }
+    `);
+    const query = introspectionFromSchema(schema);
+    const clientSchema = buildClientSchema(query);
+    const result = await plugin(
+      clientSchema,
+      [],
+      {
+        schema: 'yup',
+        scalars: {
+          ID: 'string',
+        },
+      },
+      {}
+    );
+    const wantContain = dedent`
+    export function QueryInputSchema(): yup.ObjectSchema<QueryInput> {
+      return yup.object({
+        _dummy: TestSchema.nullable().optional()
+      })
+    }`;
+    expect(result.content).toContain(wantContain);
+  });
 });
diff --git a/tests/zod.spec.ts b/tests/zod.spec.ts
index 68f223da..67a676c2 100644
--- a/tests/zod.spec.ts
+++ b/tests/zod.spec.ts
@@ -1,4 +1,5 @@
-import { buildSchema } from 'graphql';
+import { getCachedDocumentNodeFromSchema } from '@graphql-codegen/plugin-helpers';
+import { buildClientSchema, buildSchema, introspectionFromSchema, isSpecifiedScalarType } from 'graphql';
 import { dedent } from 'ts-dedent';
 
 import { plugin } from '../src/index';
@@ -989,4 +990,41 @@ describe('zod', () => {
       expect(result.content).not.toContain(wantNotContain);
     }
   });
+
+  it('issue #394', async () => {
+    const schema = buildSchema(/* GraphQL */ `
+      enum Test {
+        A
+        B
+      }
+
+      type Query {
+        _dummy: Test
+      }
+
+      input QueryInput {
+        _dummy: Test
+      }
+    `);
+    const query = introspectionFromSchema(schema);
+    const clientSchema = buildClientSchema(query);
+    const result = await plugin(
+      clientSchema,
+      [],
+      {
+        schema: 'zod',
+        scalars: {
+          ID: 'string',
+        },
+      },
+      {}
+    );
+    const wantContain = dedent`
+    export function QueryInputSchema(): z.ZodObject<Properties<QueryInput>> {
+      return z.object({
+        _dummy: TestSchema.nullish()
+      })
+    }`;
+    expect(result.content).toContain(wantContain);
+  });
 });