Index: wp-includes/post-thumbnail-template.php
===================================================================
--- wp-includes/post-thumbnail-template.php	(revision 24764)
+++ wp-includes/post-thumbnail-template.php	(working copy)
@@ -85,6 +85,8 @@
 function get_the_post_thumbnail( $post_id = null, $size = 'post-thumbnail', $attr = '' ) {
 	$post_id = ( null === $post_id ) ? get_the_ID() : $post_id;
 	$post_thumbnail_id = get_post_thumbnail_id( $post_id );
+	// Default back to standard thumbnail dimensions if post-thumbnail size not set
+	$size = ( in_array( $size, get_intermediate_image_sizes() ) ) ? $size : 'thumbnail';
 	$size = apply_filters( 'post_thumbnail_size', $size );
 	if ( $post_thumbnail_id ) {
 		do_action( 'begin_fetch_post_thumbnail_html', $post_id, $post_thumbnail_id, $size ); // for "Just In Time" filtering of all of wp_get_attachment_image()'s filters