diff --git a/index.js b/index.js
index 796e9704..20d472d1 100644
--- a/index.js
+++ b/index.js
@@ -786,9 +786,14 @@ function parse (args, opts) {
   }
 
   function isNumber (x) {
+    if (x === null || x === undefined) return false
+    // if loaded from config, may already be a number.
     if (typeof x === 'number') return true
+    // hexadecimal.
     if (/^0x[0-9a-f]+$/i.test(x)) return true
-    return /^[-+]?(?:\d+(?:\.\d*)?|\.\d+)(e[-+]?\d+)?$/.test(x)
+    // don't treat 0123 as a number; as it drops the leading '0'.
+    if (x.length > 1 && x[0] === '0') return false
+    return /^[-]?(?:\d+(?:\.\d*)?|\.\d+)(e[-+]?\d+)?$/.test(x)
   }
 
   function isUndefined (num) {
diff --git a/test/yargs-parser.js b/test/yargs-parser.js
index 60debc5d..ee76e417 100644
--- a/test/yargs-parser.js
+++ b/test/yargs-parser.js
@@ -2770,6 +2770,24 @@ describe('yargs-parser', function () {
     argv.foo.should.equal(9.39404959509494e+22)
   })
 
+  // see: https://github.com/yargs/yargs/issues/1099
+  it('does not magically convert options with leading + to number', () => {
+    const argv = parser(['--foo', '+5550100', '--bar', '+5550100'], {
+      number: 'bar'
+    })
+    argv.foo.should.equal('+5550100')
+    argv.bar.should.equal(5550100)
+  })
+
+  // see: https://github.com/yargs/yargs/issues/1099
+  it('does not magically convert options with leading 0 to number', () => {
+    const argv = parser(['--foo', '000000', '--bar', '000000'], {
+      number: 'bar'
+    })
+    argv.foo.should.equal('000000')
+    argv.bar.should.equal(0)
+  })
+
   // see: https://github.com/yargs/yargs-parser/issues/101
   describe('dot-notation array arguments combined with string arguments', function () {
     it('parses correctly when dot-notation argument is first', function () {