diff --git a/packages/swagger2openapi/index.js b/packages/swagger2openapi/index.js
index b07e19a0..788aa43f 100644
--- a/packages/swagger2openapi/index.js
+++ b/packages/swagger2openapi/index.js
@@ -171,7 +171,7 @@ function fixUpSubSchemaExtensions(schema,parent) {
         delete schema["x-discriminator"];
         for (let entry in schema.discriminator.mapping) {
             let schemaOrRef = schema.discriminator.mapping[entry];
-            if (schemaOrRef.startsWith('#/definitions/')) {
+            if (typeof schemaOrRef === 'string' && schemaOrRef.startsWith('#/definitions/')) {
                 schema.discriminator.mapping[entry] = schemaOrRef.replace('#/definitions/','#/components/schemas/');
             }
         }