Cordova ios: cordova.exec () undefined is not a function

I am developing an iOS application using cordova (1.9). I have some custom plugins for iOS to make my application. Now I run into a problem when I call:

cordova.exec(function(){},function(){},'MyPlugin','myMethod',['parameter']);

The application throws this exception:

TypeError: 'undefined' is not a function (evaluating 'cordova.exec(function(){},function(){},'WDMessageDispatcher','sendMessage',['showCategoriesList'])')

The plugin was developed after the documentation and works, but not every time. I do not understand why. Has anyone else experienced the same error?