diff --git a/client/space_lua/runtime.ts b/client/space_lua/runtime.ts
index a947055f..55ac2f08 100644
--- a/client/space_lua/runtime.ts
+++ b/client/space_lua/runtime.ts
@@ -927,6 +927,20 @@ export class LuaTable implements ILuaSettable, ILuaGettable {
     const errSf = sf || LuaStackFrame.lostFrame;
     const ctx = sf?.astCtx ?? EMPTY_CTX;
 
+    if (key === null || key === undefined) {
+      throw new LuaRuntimeError(
+        "table index is nil",
+        errSf,
+      );
+    }
+
+    if (typeof key === "number" && isNaN(key)) {
+      throw new LuaRuntimeError(
+        "table index is NaN",
+        errSf,
+      );
+    }
+
     if (this.has(key)) {
       return this.rawSet(key, value, numType);
     }
diff --git a/client/space_lua/stdlib/table_test.lua b/client/space_lua/stdlib/table_test.lua
index 38db391b..8ef17920 100644
--- a/client/space_lua/stdlib/table_test.lua
+++ b/client/space_lua/stdlib/table_test.lua
@@ -63,6 +63,38 @@ do
     assertError(function() table.remove(t, #t + 2) end)
 end
 
+-- nil and NaN keys are forbidden
+do
+    -- nil key at assignment
+    assertError(function()
+        local t = {}
+        t[nil] = "foo"
+    end)
+
+    -- nil key in table constructor (dynamic field)
+    assertError(function()
+        local k = nil
+        local _ = { [k] = "foo" }
+    end)
+
+    -- NaN key at assignment
+    assertError(function()
+        local t = {}
+        t[0/0] = "bar"
+    end)
+
+    -- NaN key in table constructor
+    assertError(function()
+        local _ = { [0/0] = "bar" }
+    end)
+
+    -- reading with nil key is also an error
+    assertError(function()
+        local t = {}
+        return t[nil]
+    end)
+end
+
 -- Reference semantics / aliasing (tables are mutable references)
 do
     local a = { x = 1 }