Index: src/wp-includes/canonical.php
===================================================================
--- src/wp-includes/canonical.php	(revision 46091)
+++ src/wp-includes/canonical.php	(working copy)
@@ -56,7 +56,7 @@
 		}
 	}
 
-	if ( is_trackback() || is_search() || is_admin() || is_preview() || is_robots() || ( $is_IIS && ! iis7_supports_permalinks() ) ) {
+	if ( is_trackback() || is_search() || is_admin() || is_preview() || ( $is_IIS && ! iis7_supports_permalinks() ) ) {
 		return;
 	}
 
@@ -530,6 +530,11 @@
 		$redirect['host'] = $original['host'];
 	}
 
+	//Even if the permalink structure ends with a slash, remove slash robots.txt.
+	if ( is_robots() ) {
+		$redirect['path'] = untrailingslashit( $redirect['path'] );
+	}
+
 	$compare_original = array( $original['host'], $original['path'] );
 
 	if ( ! empty( $original['port'] ) ) {