diff --git a/inspect.lua b/inspect.lua
index 5319f33..1e982ca 100644
--- a/inspect.lua
+++ b/inspect.lua
@@ -19,8 +19,9 @@ local function setTerm(termlike)
 end
 
 local function prepareObject(other)
+    assert(type(other) == "table", "object has to be a table")
     local keys = {}
-    for key, _ in other do
+    for key, _ in pairs(other) do
         keys.insert(key)
     end
     table.sort(keys)