Index: wp-admin/includes/plugin.php
===================================================================
--- wp-admin/includes/plugin.php	(revision 17069)
+++ wp-admin/includes/plugin.php	(working copy)
@@ -223,8 +223,11 @@
 
 	$wp_plugins = array ();
 	$plugin_root = WP_PLUGIN_DIR;
-	if ( !empty($plugin_folder) )
-		$plugin_root .= $plugin_folder;
+	$plugin_path = $plugin_folder;
+	if ( !empty($plugin_folder) ) {
+		$plugin_root .= '/' . $plugin_folder;
+		$plugin_path = trailingslashit( $plugin_folder );
+	}
 
 	// Files in wp-content/plugins directory
 	$plugins_dir = @ opendir( $plugin_root);
@@ -240,12 +243,12 @@
 						if ( substr($subfile, 0, 1) == '.' )
 							continue;
 						if ( substr($subfile, -4) == '.php' )
-							$plugin_files[] = "$file/$subfile";
+							$plugin_files[] = "$plugin_path$file/$subfile";
 					}
 				}
 			} else {
 				if ( substr($file, -4) == '.php' )
-					$plugin_files[] = $file;
+					$plugin_files[] = $plugin_path . $file;
 			}
 		}
 	} else {