diff --git a/lib/nopt-lib.js b/lib/nopt-lib.js
index 89d269f..f6f55da 100644
--- a/lib/nopt-lib.js
+++ b/lib/nopt-lib.js
@@ -1,8 +1,9 @@
 var abbrev = require('abbrev')
 const debug = require('./debug')
 const defaultTypeDefs = require('./type-defs')
+const _typeDefault = require('./type-default')
 
-function nopt (args, { types, shorthands, typeDefs, invalidHandler }) {
+function nopt (args, { types, shorthands, typeDefs, invalidHandler, typeDefault = _typeDefault }) {
   debug(types, shorthands, args, typeDefs)
 
   var data = {}
@@ -15,7 +16,7 @@ function nopt (args, { types, shorthands, typeDefs, invalidHandler }) {
   parse(args, data, argv.remain, { typeDefs, types, shorthands })
 
   // now data is full
-  clean(data, { types, typeDefs, invalidHandler })
+  clean(data, { types, typeDefs, invalidHandler, typeDefault })
   data.argv = argv
 
   Object.defineProperty(data.argv, 'toString', {
@@ -28,15 +29,13 @@ function nopt (args, { types, shorthands, typeDefs, invalidHandler }) {
   return data
 }
 
-function clean (data, { types, typeDefs, invalidHandler }) {
-  const StringType = typeDefs.String.type
+function clean (data, { types, typeDefs, invalidHandler, typeDefault = _typeDefault }) {
   const NumberType = typeDefs.Number.type
   const ArrayType = typeDefs.Array.type
   const BooleanType = typeDefs.Boolean.type
   const DateType = typeDefs.Date.type
 
   var remove = {}
-  var typeDefault = [false, true, null, StringType, ArrayType]
 
   Object.keys(data).forEach(function (k) {
     if (k === 'argv') {
@@ -44,7 +43,8 @@ function clean (data, { types, typeDefs, invalidHandler }) {
     }
     var val = data[k]
     var isArray = Array.isArray(val)
-    var type = types[k]
+    let rawType = types[k]
+    var type = rawType
     if (!isArray) {
       val = [val]
     }
@@ -82,7 +82,14 @@ function clean (data, { types, typeDefs, invalidHandler }) {
       }
 
       if (!Object.prototype.hasOwnProperty.call(types, k)) {
-        return v
+        if (typeDefault === _typeDefault) {
+          return v
+        }
+        // if the default type has been passed in then we want to validate the
+        // unknown data key instead of bailing out earlier. we also set the raw
+        // type which is passed to the invalid handler so that it can be
+        // determined if during validation if it is unknown vs invalid
+        rawType = typeDefault
       }
 
       // allow `--no-blah` to set 'blah' to null if null is allowed
@@ -93,16 +100,16 @@ function clean (data, { types, typeDefs, invalidHandler }) {
 
       var d = {}
       d[k] = v
-      debug('prevalidated val', d, v, types[k])
-      if (!validate(d, k, v, types[k], { typeDefs })) {
+      debug('prevalidated val', d, v, rawType)
+      if (!validate(d, k, v, rawType, { typeDefs })) {
         if (invalidHandler) {
-          invalidHandler(k, v, types[k], data)
+          invalidHandler(k, v, rawType, data)
         } else if (invalidHandler !== false) {
-          debug('invalid: ' + k + '=' + v, types[k])
+          debug('invalid: ' + k + '=' + v, rawType)
         }
         return remove
       }
-      debug('validated v', d, v, types[k])
+      debug('validated v', d, v, rawType)
       return d[k]
     }).filter(function (v) {
       return v !== remove
@@ -425,4 +432,5 @@ module.exports = {
   validate,
   resolveShort,
   typeDefs: defaultTypeDefs,
+  typeDefault: _typeDefault,
 }
diff --git a/lib/nopt.js b/lib/nopt.js
index 70fd809..50e2936 100644
--- a/lib/nopt.js
+++ b/lib/nopt.js
@@ -1,5 +1,4 @@
 const lib = require('./nopt-lib')
-const defaultTypeDefs = require('./type-defs')
 
 // This is the version of nopt's API that requires setting typeDefs and invalidHandler
 // on the required `nopt` object since it is a singleton. To not do a breaking change
@@ -9,7 +8,8 @@ const defaultTypeDefs = require('./type-defs')
 
 module.exports = exports = nopt
 exports.clean = clean
-exports.typeDefs = defaultTypeDefs
+exports.typeDefs = lib.typeDefs
+exports.typeDefault = lib.typeDefault
 exports.lib = lib
 
 function nopt (types = {}, shorthands = {}, args = process.argv, slice = 2) {
@@ -17,6 +17,7 @@ function nopt (types = {}, shorthands = {}, args = process.argv, slice = 2) {
     types,
     shorthands: shorthands || {},
     typeDefs: exports.typeDefs,
+    typeDefault: exports.typeDefault,
     invalidHandler: exports.invalidHandler,
   })
 }
@@ -25,6 +26,7 @@ function clean (data, types, typeDefs = exports.typeDefs) {
   return lib.clean(data, {
     types,
     typeDefs,
+    typeDefault: exports.typeDefault,
     invalidHandler: exports.invalidHandler,
   })
 }
diff --git a/lib/type-default.js b/lib/type-default.js
new file mode 100644
index 0000000..b6a2ae6
--- /dev/null
+++ b/lib/type-default.js
@@ -0,0 +1,3 @@
+const defs = require('./type-defs')
+
+module.exports = [false, true, null, defs.String.type, defs.Array.type]
diff --git a/package.json b/package.json
index fd0c0ff..2f30b28 100644
--- a/package.json
+++ b/package.json
@@ -30,9 +30,9 @@
     "tap": "^16.3.0"
   },
   "tap": {
-    "lines": 91,
-    "branches": 87,
-    "statements": 91,
+    "lines": 92,
+    "branches": 86,
+    "statements": 92,
     "nyc-arg": [
       "--exclude",
       "tap-snapshots/**"
diff --git a/test/type-default.js b/test/type-default.js
new file mode 100644
index 0000000..dd5ea73
--- /dev/null
+++ b/test/type-default.js
@@ -0,0 +1,48 @@
+const t = require('tap')
+const nopt = require('../lib/nopt-lib.js')
+
+t.test('use other type default', (t) => {
+  const NotAllowed = Symbol('NotAllowed')
+  const Invalid = Symbol('Invalid')
+
+  const clean = (data, opts) => {
+    const invalids = []
+    nopt.clean(data, {
+      types: {
+        str: nopt.typeDefs.String.type,
+        invalid: Invalid,
+      },
+      typeDefs: {
+        ...nopt.typeDefs,
+        NotAllowed: { type: NotAllowed, validate: () => false },
+        Invalid: { type: Invalid, validate: () => false },
+      },
+      invalidHandler: (k, v, type) => invalids.push([k, v, type]),
+      ...opts,
+    })
+    return {
+      keys: Object.keys(data),
+      invalids,
+    }
+  }
+
+  t.strictSame(clean({
+    str: 'aaa',
+    invalid: 'bad',
+    unknown: 'huh?',
+  }), {
+    keys: ['str', 'unknown'],
+    invalids: [['invalid', 'bad', Invalid]],
+  }, 'invalid data is removed with clean')
+
+  t.strictSame(clean({
+    str: 'aaa',
+    invalid: 'bad',
+    unknown: 'huh?',
+  }, { typeDefault: NotAllowed }), {
+    keys: ['str'],
+    invalids: [['invalid', 'bad', Invalid], ['unknown', 'huh?', NotAllowed]],
+  }, 'invalid and unknown data is removed with a custom typeDefault')
+
+  t.end()
+})