diff --git a/src/option-base.impl.ts b/src/option-base.impl.ts
index cd6ec28..ac0bef7 100644
--- a/src/option-base.impl.ts
+++ b/src/option-base.impl.ts
@@ -59,8 +59,8 @@ export class ZOptionBase<TOption extends ZOption> implements ZOption {
     return this._impl.optionLocation(init);
   }
 
-  supportedOptions(): Iterable<string> {
-    return this._impl.optionsMeta().keys();
+  supportedOptions(): readonly string[] {
+    return [...this._impl.optionsMeta().keys()];
   }
 
   optionMeta(key: string): ZOptionMeta.Combined {
diff --git a/src/option-syntax.ts b/src/option-syntax.ts
index 4643966..025d293 100644
--- a/src/option-syntax.ts
+++ b/src/option-syntax.ts
@@ -17,7 +17,7 @@ export type ZOptionSyntax =
 /**
  * @param args  An array of command line arguments to process.
  *
- * @returns An iterable of option inputs. May be empty if the command line argument has another syntax.
+ * @returns A read-only array of option inputs. May be empty if the command line argument has another syntax.
  */
     (this: void, args: readonly [string, ...string[]]) => readonly ZOptionInput[];
 
diff --git a/src/option.ts b/src/option.ts
index fa66ddc..d498ecd 100644
--- a/src/option.ts
+++ b/src/option.ts
@@ -143,9 +143,9 @@ export interface ZOption {
   /**
    * Lists all options supported by the parser.
    *
-   * @returns An iterable of all supported option keys except hidden ones.
+   * @returns A read-only array of all supported option keys except hidden ones.
    */
-  supportedOptions(): Iterable<string>;
+  supportedOptions(): readonly string[];
 
   /**
    * Returns meta information for option.