diff --git a/src/benchmark.js b/src/benchmark.js
index 933fc12..a71fc40 100644
--- a/src/benchmark.js
+++ b/src/benchmark.js
@@ -29,7 +29,12 @@ import {
 } from './reporter/terminal/index.js'
 import { runtime } from './runtime.js'
 import { now } from './time.js'
-import { isAsyncFnResource, isFunction, isObject } from './utils.js'
+import {
+  isAsyncFnResource,
+  isAsyncFunction,
+  isFunction,
+  isObject,
+} from './utils.js'
 
 let groupName = null
 const groups = new Map()
@@ -75,6 +80,10 @@ export function group(name, cb = undefined) {
   }
   if (!isFunction(cb))
     throw new TypeError(`expected function, got ${cb.constructor.name}`)
+  if (isAsyncFunction(cb))
+    throw new TypeError(
+      `expected synchronous function, got asynchronous function ${cb.constructor.name}`
+    )
   if (isObject(name)) {
     if (name.name != null && 'string' !== typeof name.name)
       throw new TypeError(
@@ -127,14 +136,8 @@ export function group(name, cb = undefined) {
       before: name.before ?? emptyFunction,
       after: name.after ?? emptyFunction,
     })
-  if (isAsyncFnResource(cb)) {
-    cb().then(() => {
-      groupName = null
-    })
-  } else {
-    cb()
-    groupName = null
-  }
+  cb()
+  groupName = null
 }
 
 /**
@@ -362,7 +365,7 @@ export async function run(opts = {}) {
       if (once) log('')
       log(groupHeader(groupName, opts))
     }
-    isAsyncFnResource(groupOpts.before)
+    isAsyncFunction(groupOpts.before)
       ? await groupOpts.before()
       : groupOpts.before()
 
@@ -372,7 +375,7 @@ export async function run(opts = {}) {
       opts,
       groupOpts
     )
-    isAsyncFnResource(groupOpts.after)
+    isAsyncFunction(groupOpts.after)
       ? await groupOpts.after()
       : groupOpts.after()
   }
diff --git a/src/index.d.ts b/src/index.d.ts
index f23dc7d..13f6710 100644
--- a/src/index.d.ts
+++ b/src/index.d.ts
@@ -9,12 +9,9 @@ export type GroupOptions = {
   after?: () => void | Promise<void>
 }
 
-export function group(cb: () => void | Promise<void>): void
-export function group(name: string, cb: () => void | Promise<void>): void
-export function group(
-  options: GroupOptions,
-  cb: () => void | Promise<void>
-): void
+export function group(cb: () => void): void
+export function group(name: string, cb: () => void): void
+export function group(options: GroupOptions, cb: () => void): void
 
 export type BenchmarkOptions = {
   samples?: number // minimum number of samples
diff --git a/src/lib.js b/src/lib.js
index 74d701f..085f198 100644
--- a/src/lib.js
+++ b/src/lib.js
@@ -22,6 +22,7 @@ import {
   AsyncFunction,
   checkDividend,
   isAsyncFnResource,
+  isAsyncFunction,
   isFunction,
   isObject,
 } from './utils.js'
@@ -216,8 +217,8 @@ export async function measure(fn, opts = {}) {
   opts.before = opts.before ?? emptyFunction
   opts.after = opts.after ?? emptyFunction
 
-  const asyncBefore = isAsyncFnResource(opts.before)
-  const asyncAfter = isAsyncFnResource(opts.after)
+  const asyncBefore = isAsyncFunction(opts.before)
+  const asyncAfter = isAsyncFunction(opts.after)
 
   const asyncFunction = opts.async || asyncBefore || asyncAfter
 
diff --git a/src/utils.js b/src/utils.js
index 39682aa..41f0aad 100644
--- a/src/utils.js
+++ b/src/utils.js
@@ -27,7 +27,7 @@ export const AsyncFunction = (async () => {}).constructor
  * @param {Function} fn the function to check
  * @returns {Boolean} true if the function is an async function
  */
-const isAsyncFunction = fn => {
+export const isAsyncFunction = fn => {
   return AsyncFunction === fn?.constructor
 }
 
@@ -49,7 +49,7 @@ export const isAsyncFnResource = fn => {
     const promiseLike = isPromiseLike(fnCall)
     if (promiseLike) {
       // silence promise rejection
-      fnCall.catch(() => {})
+      fnCall.then(() => {}).catch(() => {})
     }
     return promiseLike
   } catch {