diff --git a/packages/typeless-form/package.json b/packages/typeless-form/package.json
index 87c6f81..120aaf3 100644
--- a/packages/typeless-form/package.json
+++ b/packages/typeless-form/package.json
@@ -67,7 +67,7 @@
     "rimraf": "^3.0.2",
     "rxjs": "^6.3.3",
     "tslint": "6.0.0",
-    "typescript": "3.8.2"
+    "typescript": "^3.9.6"
   },
   "resolutions": {
     "@types/react": "^16.8.12"
diff --git a/packages/typeless-router/package.json b/packages/typeless-router/package.json
index 05be25c..3c2fc9f 100644
--- a/packages/typeless-router/package.json
+++ b/packages/typeless-router/package.json
@@ -67,7 +67,7 @@
     "rimraf": "^3.0.2",
     "rxjs": "^6.3.3",
     "tslint": "6.0.0",
-    "typescript": "3.8.2"
+    "typescript": "^3.9.6"
   },
   "resolutions": {
     "@types/react": "^16.8.12"
diff --git a/packages/typeless/__tests__/type/Epic.test.ts b/packages/typeless/__tests__/type/Epic.test.ts
new file mode 100644
index 0000000..edc33a3
--- /dev/null
+++ b/packages/typeless/__tests__/type/Epic.test.ts
@@ -0,0 +1,42 @@
+import { createModule, ActionMap } from '../../src/createModule';
+import { Epic } from '../../src/Epic';
+import { TT } from './TypeTester';
+
+function createAction<T extends ActionMap>(acts: T) {
+  return createModule(Symbol('test')).withActions(acts)[1];
+}
+TT.describe('onMany', () => {
+  const m1 = createAction({ foo: (num: number) => ({ payload: { num } }) });
+  const m2 = createAction({ foo: (boo: boolean) => ({ payload: { boo } }) });
+  const m3 = createAction({ foo: () => ({ payload: 'bar' }) });
+  const m4 = createAction({ foo: () => ({ payload: Symbol('baz') }) });
+  const m5 = createAction({ foo: (str: string) => ({ payload: { str } }) });
+  const m6 = createAction({ foo: () => ({ payload: ['array'] }) });
+
+  TT.describe('should error called with', () => {
+    TT.it('empty array', () => {
+      // @ts-expect-error
+      new Epic().onMany([], () => null);
+    });
+    TT.it('has 1 item array', () => {
+      // @ts-expect-error
+      new Epic().onMany([m1.foo], () => null);
+    });
+  });
+  TT.describe("should infer callback's arguments", () => {
+    TT.it('called with many arguments', () => {
+      new Epic().onMany([m1.foo, m2.foo, m3.foo, m4.foo, m5.foo, m6.foo], p => {
+        type Expected =
+          | { num: number }
+          | { boo: boolean }
+          | string
+          | symbol
+          | { str: string }
+          | string[];
+        TT.assert<TT.Eq<typeof p, Expected>>();
+
+        return null;
+      });
+    });
+  });
+});
diff --git a/packages/typeless/__tests__/type/useMappedState.test.ts b/packages/typeless/__tests__/type/useMappedState.test.ts
index 76578e4..41a95a8 100644
--- a/packages/typeless/__tests__/type/useMappedState.test.ts
+++ b/packages/typeless/__tests__/type/useMappedState.test.ts
@@ -10,6 +10,10 @@ TT.describe('useMappedState', () => {
   const getCountState = createTestModule<{ count: number }>();
   const getTextState = createTestModule<{ text: string }>();
 
+  TT.it('should error with empty array', () => {
+    // @ts-expect-error
+    useMappedState([], () => null);
+  });
   TT.it("should infer callback's arguments type", () => {
     useMappedState([getCountState, getTextState], (c, t) => {
       TT.assert<TT.Eq<typeof c, { count: number }>>();
diff --git a/packages/typeless/package.json b/packages/typeless/package.json
index 88f7014..d6364dc 100644
--- a/packages/typeless/package.json
+++ b/packages/typeless/package.json
@@ -69,7 +69,7 @@
     "shallow-equal": "^1.2.1",
     "ts-jest": "^25.5.0",
     "tslint": "6.0.0",
-    "typescript": "3.8.2"
+    "typescript": "^3.9.6"
   },
   "peerDependencies": {
     "react": "^16.8.2",
diff --git a/packages/typeless/src/Epic.ts b/packages/typeless/src/Epic.ts
index 6d56a84..ff64e90 100644
--- a/packages/typeless/src/Epic.ts
+++ b/packages/typeless/src/Epic.ts
@@ -43,29 +43,7 @@ export class Epic {
   on<TAC extends AC>(ac: TAC, handler: EpicHandler<TAC>) {
     return this.add(ac, handler);
   }
-  onMany<TAC extends AC, TAC2 extends AC>(
-    ac: [TAC, TAC2],
-    handler: EpicHandler<TAC | TAC2>
-  ): this;
-  onMany<TAC extends AC, TAC2 extends AC, TAC3 extends AC>(
-    ac: [TAC, TAC2, TAC3],
-    handler: EpicHandler<TAC | TAC2 | TAC3>
-  ): this;
-  onMany<TAC extends AC, TAC2 extends AC, TAC3 extends AC, TAC4 extends AC>(
-    ac: [TAC, TAC2, TAC3, TAC4],
-    handler: EpicHandler<TAC | TAC2 | TAC3 | TAC4>
-  ): this;
-  onMany<
-    TAC extends AC,
-    TAC2 extends AC,
-    TAC3 extends AC,
-    TAC4 extends AC,
-    TAC5 extends AC
-  >(
-    ac: [TAC, TAC2, TAC3, TAC4, TAC5],
-    handler: EpicHandler<TAC | TAC2 | TAC3 | TAC4 | TAC5>
-  ): this;
-  onMany(ac: AC[], handler: EpicHandler<AC>) {
+  onMany<T extends [AC, AC, ...AC[]]>(ac: T, handler: EpicHandler<T[number]>) {
     return this.add(ac, handler);
   }
 
diff --git a/packages/typeless/src/types.ts b/packages/typeless/src/types.ts
index fefd1c6..e1de918 100644
--- a/packages/typeless/src/types.ts
+++ b/packages/typeless/src/types.ts
@@ -31,6 +31,6 @@ export interface StateGetter<T> {
   useState(): T;
 }
 
-export type TupleOfStateGetter = [] | [StateGetter<any>, ...StateGetter<any>[]];
+export type TupleOfStateGetter = [StateGetter<any>, ...StateGetter<any>[]];
 
 export type EqualityFn<T> = (a: T, b: T) => boolean;
diff --git a/yarn.lock b/yarn.lock
index 52f48de..b8e80f9 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -7570,10 +7570,10 @@ typedarray@^0.0.6:
   resolved "https://registry.yarnpkg.com/typedarray/-/typedarray-0.0.6.tgz#867ac74e3864187b1d3d47d996a78ec5c8830777"
   integrity sha1-hnrHTjhkGHsdPUfZlqeOxciDB3c=
 
-typescript@3.8.2:
-  version "3.8.2"
-  resolved "https://registry.yarnpkg.com/typescript/-/typescript-3.8.2.tgz#91d6868aaead7da74f493c553aeff76c0c0b1d5a"
-  integrity sha512-EgOVgL/4xfVrCMbhYKUQTdF37SQn4Iw73H5BgCrF1Abdun7Kwy/QZsE/ssAy0y4LxBbvua3PIbFsbRczWWnDdQ==
+typescript@^3.9.6:
+  version "3.9.6"
+  resolved "https://registry.yarnpkg.com/typescript/-/typescript-3.9.6.tgz#8f3e0198a34c3ae17091b35571d3afd31999365a"
+  integrity sha512-Pspx3oKAPJtjNwE92YS05HQoY7z2SFyOpHo9MqJor3BXAGNaPUs83CuVp9VISFkSjyRfiTpmKuAYGJB7S7hOxw==
 
 uglify-js@^3.1.4:
   version "3.8.0"