diff --git a/src/index.ts b/src/index.ts
index 8fd944a..c64f8f9 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -52,7 +52,7 @@ formatsPlugin.get = (name: FormatName, mode: FormatMode = "full"): Format => {
 }
 
 function addFormats(ajv: Ajv, list: FormatName[], fs: DefinedFormats, exportName: Name): void {
-  ajv.opts.code.formats ??= _`require("ajv-formats/dist/formats").${exportName}`
+  ajv.opts?.code.formats ??= _`require("ajv-formats/dist/formats").${exportName}`
   for (const f of list) ajv.addFormat(f, fs[f])
 }