diff --git a/src/assertions/prop.js b/src/assertions/prop.js
index <HASH>..<HASH> 100644
--- a/src/assertions/prop.js
+++ b/src/assertions/prop.js
@@ -3,21 +3,21 @@ import should from 'should';
 
 const Assertion = should.Assertion;
 
-Assertion.add('prop', function(expectedKey, expectedValue){
+Assertion.add('prop', function (expectedKey, expectedValue) {
   const wrapper = WrapperBuilder(this.obj),
-  wrapperProp = wrapper.prop(expectedKey);
+    wrapperProp = wrapper.prop(expectedKey);
 
-  if(arguments.length > 1 && typeof wrapperProp !== 'undefined') {
+  if (arguments.length > 1 && typeof wrapperProp !== 'undefined') {
     this.params = {
       actual: wrapper.name(),
       operator: `prop '${expectedKey}' to have value ${should.format(expectedValue)}, instead found ${should.format(wrapperProp)}`
     };
     should(wrapperProp).be.eql(expectedValue, ' ');
-  }else{
+  } else {
     this.params = {
       actual: wrapper.name(),
       operator: `to have prop '${expectedKey}'`
     };
-    should(wrapperProp).not.be.undefined(' ');
+    should(wrapperProp).not.be.undefined();
   }
 }); 
\ No newline at end of file
No args needed for undefined() assert

Errors thrown because no args needed to be passed into the undefined() assert.

Type checking has recently been added to should.js using TypeScript.