diff --git a/CHANGELOG.md b/CHANGELOG.md
index 0fabbc0d..54dee9e0 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,6 +1,7 @@
 [Read translated version (en)](./translations/en/CHANGELOG.md)
 
 # 未リリース分
+- 関数の省略された引数は`exists`でfalseを返すように
 
 # 0.18.0
 - `Core:abort`でプログラムを緊急停止できるように
diff --git a/etc/aiscript.api.md b/etc/aiscript.api.md
index e06fa825..b35a710b 100644
--- a/etc/aiscript.api.md
+++ b/etc/aiscript.api.md
@@ -23,7 +23,7 @@ type AddAssign_2 = NodeBase_2 & {
 };
 
 // @public (undocumented)
-export const AISCRIPT_VERSION: "0.18.0";
+export const AISCRIPT_VERSION: "0.19.0";
 
 // @public (undocumented)
 abstract class AiScriptError extends Error {
diff --git a/src/interpreter/index.ts b/src/interpreter/index.ts
index 23df41ea..5863853c 100644
--- a/src/interpreter/index.ts
+++ b/src/interpreter/index.ts
@@ -236,8 +236,8 @@ export class Interpreter {
 			return result ?? NULL;
 		} else {
 			const _args = new Map<string, Variable>();
-			for (let i = 0; i < (fn.args ?? []).length; i++) {
-				_args.set(fn.args![i]!, {
+			if (fn.args) for (let i = 0; i < fn.args.length; i++) {
+				if (args[i]) _args.set(fn.args[i]!, {
 					isMutable: true,
 					value: args[i]!,
 				});
diff --git a/test/index.ts b/test/index.ts
index 290efa73..08104a54 100644
--- a/test/index.ts
+++ b/test/index.ts
@@ -1296,6 +1296,27 @@ describe('Function call', () => {
 		}
 		assert.fail();
 	});
+
+	test.concurrent('exists on unprovided arg', async () => {
+		const res = await exe(`
+		@f(x) { return exists x }
+		<: f()
+		`);
+		eq(res, FALSE);
+	});
+
+	test.concurrent('accessing unprovided arg', async () => {
+		try {
+			await exe(`
+			@f(x) { return x }
+			<: f()
+			`);
+		} catch (e) {
+			assert.ok(e instanceof AiScriptRuntimeError);
+			return;
+		}
+		assert.fail();
+	});
 });
 
 describe('Return', () => {