Index: src/wp-includes/class-wp-image-editor-imagick.php
===================================================================
--- src/wp-includes/class-wp-image-editor-imagick.php	(revision 39548)
+++ src/wp-includes/class-wp-image-editor-imagick.php	(working copy)
@@ -149,7 +149,7 @@
 			$file_parts = pathinfo( $this->file );
 			$filename = $this->file;
 
-			if ( 'pdf' == strtolower( $file_parts['extension'] ) ) {
+			if ( ! empty( $file_parts['extension'] ) && 'pdf' == strtolower( $file_parts['extension'] ) ) {
 				$filename = $this->pdf_setup();
 			}