Custom block type problem

Pls tell me what i am doing wrong:

My custom block types save function:
(the var props.attributes.shortcode contains: ‘[photo 5594]’)

save: 	function( props ) {
			var shortcode = props.attributes.shortcode || '';

			if ( shortcode.length ) {
				return el( 'p', null, shortcode );
			}
		},

The content of the page in the classic editor is:

<!-- wp:wppa/gutenberg-photo -->
<p class="wp-block-wppa-gutenberg-photo">[photo 5594]</p>
<!-- /wp:wppa/gutenberg-photo -->

So far it works, but re-editing the post produces the error:
This block contains unexpected or invalid content.

Clicking the resolve button he wants to change:

Current
<p class="wp-block-wppa-gutenberg-photo">[photo 5594]</p>

into:

After Conversion
<p class="wp-block-wppa-gutenberg-photo">[photo 5594]</p>