grunt copy - Couldn't load the png image

I am new to grunt. I am copying the src folder to build by running the command grunt copy. But I can't open all png images in the build folder(destination folder). I got a fatal error while trying to open the image. Errors: "Could not load image" "Fatal Error reading PNG image file: Not a PNG file"

Grunt versions I am using: grunt-cli v1.2.0, grunt v0.4.5, grunt-contrib-copy@1.0.0.

I have tried with all available links in stack overflow and other sites.

Kindly help me.

I got the answer;

I've done few modifications in the gruntfile.js:

• Added grunt.loadNpmTasks('grunt-contrib-copy'); to the end of gruntfile.js.

• Modified the options like the following:

  options: { punctuation: '' }