Index: wp-includes/class-wp-image-editor-gd.php
===================================================================
--- wp-includes/class-wp-image-editor-gd.php	(revision 26816)
+++ wp-includes/class-wp-image-editor-gd.php	(working copy)
@@ -205,13 +205,16 @@
 		$orig_size = $this->size;
 
 		foreach ( $sizes as $size => $size_data ) {
-			if ( ! ( isset( $size_data['width'] ) && isset( $size_data['height'] ) ) )
+			// that we don't have NULL or 0 for both values
+			if ( 0 == ( absint( $size_data['width'] ) + absint( $size_data['height'] ) ) ){ 
 				continue;
+			}
 
-			if ( ! isset( $size_data['crop'] ) )
+			if ( ! isset( $size_data['crop'] ) ){
 				$size_data['crop'] = false;
+			}
 
-			$image = $this->_resize( $size_data['width'], $size_data['height'], $size_data['crop'] );
+			$image = $this->_resize( absint( $size_data['width'] ), absint( $size_data['height'] ), $size_data['crop'] );
 
 			if( ! is_wp_error( $image ) ) {
 				$resized = $this->_save( $image );
Index: wp-includes/class-wp-image-editor-imagick.php
===================================================================
--- wp-includes/class-wp-image-editor-imagick.php	(revision 26816)
+++ wp-includes/class-wp-image-editor-imagick.php	(working copy)
@@ -272,13 +272,16 @@
 			if ( ! $this->image )
 				$this->image = $orig_image->getImage();
 
-			if ( ! ( isset( $size_data['width'] ) && isset( $size_data['height'] ) ) )
+			// that we don't have NULL or 0 for both values
+			if ( 0 == ( absint( $size_data['width'] ) + absint( $size_data['height'] ) ) ){ 
 				continue;
+			}
 
-			if ( ! isset( $size_data['crop'] ) )
+			if ( ! isset( $size_data['crop'] ) ){
 				$size_data['crop'] = false;
+			}
 
-			$resize_result = $this->resize( $size_data['width'], $size_data['height'], $size_data['crop'] );
+			$resize_result = $this->resize( absint( $size_data['width'] ), absint( $size_data['height'] ), $size_data['crop'] );
 
 			if( ! is_wp_error( $resize_result ) ) {
 				$resized = $this->_save( $this->image );