diff --git a/lib/internal/validators.js b/lib/internal/validators.js
index 6abe332fc5fdc0..9bca0aa747725e 100644
--- a/lib/internal/validators.js
+++ b/lib/internal/validators.js
@@ -229,6 +229,11 @@ const validateFunction = hideStackFrames((value, name) => {
     throw new ERR_INVALID_ARG_TYPE(name, 'Function', value);
 });
 
+const validateUndefined = hideStackFrames((value, name) => {
+  if (value !== undefined)
+    throw new ERR_INVALID_ARG_TYPE(name, 'undefined', value);
+});
+
 module.exports = {
   isInt32,
   isUint32,
@@ -247,6 +252,7 @@ module.exports = {
   validateSignalName,
   validateString,
   validateUint32,
+  validateUndefined,
   validateCallback,
   validateAbortSignal,
 };
diff --git a/lib/wasi.js b/lib/wasi.js
index 63209ce716cb9a..43ecf94b4cb050 100644
--- a/lib/wasi.js
+++ b/lib/wasi.js
@@ -21,6 +21,7 @@ const {
   validateFunction,
   validateInt32,
   validateObject,
+  validateUndefined,
 } = require('internal/validators');
 const { WASI: _WASI } = internalBinding('wasi');
 const kExitCode = Symbol('kExitCode');
@@ -120,10 +121,7 @@ class WASI {
     const { _start, _initialize } = this[kInstance].exports;
 
     validateFunction(_start, 'instance.exports._start');
-    if (_initialize !== undefined) {
-      throw new ERR_INVALID_ARG_TYPE(
-        'instance.exports._initialize', 'undefined', _initialize);
-    }
+    validateUndefined(_initialize, 'instance.exports._initialize');
 
     try {
       _start();
@@ -147,16 +145,9 @@ class WASI {
 
     const { _start, _initialize } = this[kInstance].exports;
 
-    if (typeof _initialize !== 'function' && _initialize !== undefined) {
-      throw new ERR_INVALID_ARG_TYPE(
-        'instance.exports._initialize', 'function', _initialize);
-    }
-    if (_start !== undefined) {
-      throw new ERR_INVALID_ARG_TYPE(
-        'instance.exports._start', 'undefined', _initialize);
-    }
-
+    validateUndefined(_start, 'instance.exports._start');
     if (_initialize !== undefined) {
+      validateFunction(_initialize, 'instance.exports._initialize');
       _initialize();
     }
   }
diff --git a/test/wasi/test-wasi-initialize-validation.js b/test/wasi/test-wasi-initialize-validation.js
index 79b0bd8485a483..40dfd864d1874e 100644
--- a/test/wasi/test-wasi-initialize-validation.js
+++ b/test/wasi/test-wasi-initialize-validation.js
@@ -78,7 +78,8 @@ const bufferSource = fixtures.readSync('simple.wasm');
       () => { wasi.initialize(instance); },
       {
         code: 'ERR_INVALID_ARG_TYPE',
-        message: /"instance\.exports\._start" property must be undefined/
+        message: 'The "instance.exports._start" property must be' +
+          ' undefined. Received function _start',
       }
     );
   }
diff --git a/test/wasi/test-wasi-start-validation.js b/test/wasi/test-wasi-start-validation.js
index 5c6a1ede5d4fd7..2059ff081e88dd 100644
--- a/test/wasi/test-wasi-start-validation.js
+++ b/test/wasi/test-wasi-start-validation.js
@@ -78,7 +78,8 @@ const bufferSource = fixtures.readSync('simple.wasm');
       () => { wasi.start(instance); },
       {
         code: 'ERR_INVALID_ARG_TYPE',
-        message: /"instance\.exports\._initialize" property must be undefined/
+        message: 'The "instance.exports._initialize" property must be' +
+          ' undefined. Received function _initialize',
       }
     );
   }