Index: src/wp-includes/general-template.php
===================================================================
--- src/wp-includes/general-template.php	(revision 36223)
+++ src/wp-includes/general-template.php	(working copy)
@@ -914,6 +914,11 @@
 	 * @param string $sep Document title separator. Default '-'.
 	 */
 	$sep = apply_filters( 'document_title_separator', '-' );
+	if ( empty( $sep ) ) {
+		$join = ' ';
+	} else {
+		$join = " $sep ";
+	}
 
 	/**
 	 * Filter the parts of the document title.
@@ -931,7 +936,7 @@
 	 */
 	$title = apply_filters( 'document_title_parts', $title );
 
-	$title = implode( " $sep ", array_filter( $title ) );
+	$title = implode( $join, array_filter( $title ) );
 	$title = wptexturize( $title );
 	$title = convert_chars( $title );
 	$title = esc_html( $title );