diff --git a/lstrlib.c b/lstrlib.c
index e9c60c0f..dde868c0 100644
--- a/lstrlib.c
+++ b/lstrlib.c
@@ -1180,8 +1180,7 @@ static int str_format (lua_State *L) {
           break;
         }
         default: {  /* also treat cases 'pnLlh' */
-          return luaL_error(L, "invalid option '%%%c' to 'format'",
-                               *(strfrmt - 1));
+          return luaL_error(L, "invalid conversion '%s' to 'format'", form);
         }
       }
       lua_assert(nb < MAX_ITEM);
diff --git a/testes/strings.lua b/testes/strings.lua
index 1260dbcc..587a0e06 100644
--- a/testes/strings.lua
+++ b/testes/strings.lua
@@ -302,7 +302,7 @@ check("%100.3d", "too long")
 check("%1"..aux..".3d", "too long")
 check("%1.100d", "too long")
 check("%10.1"..aux.."004d", "too long")
-check("%t", "invalid option")
+check("%t", "invalid conversion")
 check("%"..aux.."d", "repeated flags")
 check("%d %d", "no value")