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Im having this issue and I dont know why?

Can anyone suggest where to start looking.

I found one jquery issue

Uncaught TypeError: Cannot read property ‘top’ of undefined

in this folder /js/functions.js?ver=20150315 line 106

this is the code from that section

/*
		 * Fixed header for large screen.
		 * If the header becomes more than 48px tall, unfix the header.
		 *
		 * The callback on the scroll event is only added if there is a header
		 * image and we are not on mobile.
		 */
		if ( _window.width() > 781 ) {
			var mastheadHeight = $( '#masthead' ).height(),
				toolbarOffset, mastheadOffset;

			if ( mastheadHeight > 48 ) {
				body.removeClass( 'masthead-fixed' );
			}

			if ( body.is( '.header-image' ) ) {
				toolbarOffset  = body.is( '.admin-bar' ) ? $( '#wpadminbar' ).height() : 0;
				mastheadOffset = $( '#masthead' ).offset().top - toolbarOffset;

				_window.on( 'scroll.twentyfourteen', function() {
					if ( _window.scrollTop() > mastheadOffset && mastheadHeight < 49 ) {
						body.addClass( 'masthead-fixed' );
					} else {
						body.removeClass( 'masthead-fixed' );
					}
				} );
			}
		}

Im grateful for any help any one can offer

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