diff --git a/.changeset/serious-days-live.md b/.changeset/serious-days-live.md
new file mode 100644
index 000000000..0036fc425
--- /dev/null
+++ b/.changeset/serious-days-live.md
@@ -0,0 +1,5 @@
+---
+"openapi-typescript": patch
+---
+
+Make sure result is an object before using the in operator.
diff --git a/packages/openapi-typescript/src/transform/schema-object.ts b/packages/openapi-typescript/src/transform/schema-object.ts
index 55e58ee5b..0885324b3 100644
--- a/packages/openapi-typescript/src/transform/schema-object.ts
+++ b/packages/openapi-typescript/src/transform/schema-object.ts
@@ -269,7 +269,7 @@ function transformSchemaObjectCore(schemaObject: SchemaObject, options: Transfor
   if ("type" in schemaObject && schemaObject.type) {
     if (typeof options.ctx.transform === "function") {
       const result = options.ctx.transform(schemaObject, options);
-      if (result) {
+      if (result && typeof result === "object") {
         if ("schema" in result) {
           if (result.questionToken) {
             return ts.factory.createUnionTypeNode([result.schema, UNDEFINED]);
@@ -470,7 +470,7 @@ function transformSchemaObjectCore(schemaObject: SchemaObject, options: Transfor
 
         if (typeof options.ctx.transform === "function") {
           const result = options.ctx.transform(v as SchemaObject, options);
-          if (result) {
+          if (result && typeof result === "object") {
             if ("schema" in result) {
               type = result.schema;
               optional = result.questionToken ? QUESTION_TOKEN : optional;