diff --git a/spec/modifiers/custom.spec.ts b/spec/modifiers/custom.spec.ts
new file mode 100644
index 0000000..59547ec
--- /dev/null
+++ b/spec/modifiers/custom.spec.ts
@@ -0,0 +1,48 @@
+import { z } from 'zod';
+import { expectSchema } from '../lib/helpers';
+
+// File as a class is not available on older node versions
+// so I am defining this just for testing purposes
+class File {}
+
+describe('custom', () => {
+  it('generates OpenAPI schema for custom type', () => {
+    const FileSchema = z
+      .custom(target => target instanceof File)
+      .openapi({
+        type: 'string',
+        format: 'binary',
+      })
+      .openapi('File');
+
+    expectSchema([FileSchema], {
+      File: {
+        type: 'string',
+        format: 'binary',
+      },
+    });
+  });
+
+  it('generates OpenAPI schema for custom type in object', () => {
+    const FileUploadSchema = z
+      .object({
+        file: z
+          .custom(target => target instanceof File)
+          .openapi({
+            type: 'string',
+            format: 'binary',
+          }),
+      })
+      .openapi('FileUpload');
+
+    expectSchema([FileUploadSchema], {
+      FileUpload: {
+        type: 'object',
+        properties: {
+          file: { type: 'string', format: 'binary' },
+        },
+        required: ['file'],
+      },
+    });
+  });
+});
diff --git a/spec/modifiers/instanceof.spec.ts b/spec/modifiers/instanceof.spec.ts
new file mode 100644
index 0000000..7fb9763
--- /dev/null
+++ b/spec/modifiers/instanceof.spec.ts
@@ -0,0 +1,46 @@
+import { z } from 'zod';
+import { expectSchema } from '../lib/helpers';
+
+// File as a class is not available on older node versions
+// so I am defining this just for testing purposes
+class File {}
+
+describe('instanceof', () => {
+  it('generates OpenAPI schema for instanceof type', () => {
+    const FileSchema = z
+      .instanceof(File)
+      .openapi({
+        type: 'string',
+        format: 'binary',
+      })
+      .openapi('File');
+
+    expectSchema([FileSchema], {
+      File: {
+        type: 'string',
+        format: 'binary',
+      },
+    });
+  });
+
+  it('generates OpenAPI schema for instanceof type in object', () => {
+    const FileUploadSchema = z
+      .object({
+        file: z.instanceof(File).openapi({
+          type: 'string',
+          format: 'binary',
+        }),
+      })
+      .openapi('FileUpload');
+
+    expectSchema([FileUploadSchema], {
+      FileUpload: {
+        type: 'object',
+        properties: {
+          file: { type: 'string', format: 'binary' },
+        },
+        required: ['file'],
+      },
+    });
+  });
+});
diff --git a/spec/modifiers/refine.spec.ts b/spec/modifiers/refine.spec.ts
index 739455c..b451200 100644
--- a/spec/modifiers/refine.spec.ts
+++ b/spec/modifiers/refine.spec.ts
@@ -64,7 +64,7 @@ describe('refine', () => {
       [
         z
           .object({
-            test: z.onumber().refine(num => num && num % 2 === 0),
+            test: z.onumber().refine(num => !num || num % 2 === 0),
           })
           .openapi('ObjectWithRefinedString'),
       ],
diff --git a/src/metadata.ts b/src/metadata.ts
index 56f264c..05f1158 100644
--- a/src/metadata.ts
+++ b/src/metadata.ts
@@ -151,10 +151,6 @@ export class Metadata {
   }
 
   static isOptionalSchema(zodSchema: ZodTypeAny): boolean {
-    if (isZodType(zodSchema, 'ZodEffects')) {
-      return this.isOptionalSchema(zodSchema._def.schema);
-    }
-
     return zodSchema.isOptional();
   }
 }