diff --git a/client/space_lua/stdlib/math.ts b/client/space_lua/stdlib/math.ts
index bffaf8e5..eb949a68 100644
--- a/client/space_lua/stdlib/math.ts
+++ b/client/space_lua/stdlib/math.ts
@@ -42,22 +42,23 @@ export const mathApi = new LuaTable({
   }),
 
   /**
-   * If the value x is representable as a Lua integer, returns an
-   * integer with that value. Otherwise returns nil.
+   * If the value x is representable as a Lua integer, returns an integer
+   * with that value. Otherwise returns nil.
+   * Strings are NOT accepted — only Lua number values.
    */
   tointeger: new LuaBuiltinFunction((_sf, x?: any) => {
-    if (x === undefined || x === null) return null;
     if (typeof x === "number") {
-      // plain integer
-      if (Number.isInteger(x)) return x;
-      return null;
+      return Number.isInteger(x) && isFinite(x) ? x : null;
     }
     if (isTaggedFloat(x)) {
-      // float with integer value e.g. 3.0
-      if (Number.isInteger(x.value)) return x.value;
-      return null;
+      const n = x.value;
+      return Number.isInteger(n) && isFinite(n) ? n : null;
+    }
+    if (typeof x === "string") {
+      const n = untagNumber(x); // Number(x) coerces the string
+      if (isNaN(n) || !isFinite(n) || !Number.isInteger(n)) return null;
+      return n;
     }
-    if (typeof x === "bigint") return x;
     return null;
   }),
 
diff --git a/client/space_lua/stdlib/math_test.lua b/client/space_lua/stdlib/math_test.lua
index 37a877f9..b6911337 100644
--- a/client/space_lua/stdlib/math_test.lua
+++ b/client/space_lua/stdlib/math_test.lua
@@ -189,8 +189,13 @@ do
   assertEquals(math.tointeger(3.5),  nil, "tointeger(3.5)")
   assertEquals(math.tointeger(-0.1), nil, "tointeger(-0.1)")
 
-  -- non-numbers yields nil (no error)
-  assertEquals(math.tointeger("3"),  nil, "tointeger(string)")
+  -- string coercion: integer-valued string yields integer
+  assertEquals(math.tointeger("3"),  3,   "tointeger(string int)")
+  assertEquals(math.tointeger("3.0"), 3,  "tointeger(string float whole)")
+  assertEquals(math.tointeger("3.5"), nil, "tointeger(string float frac)")
+  assertEquals(math.tointeger("x"),   nil, "tointeger(non-numeric string)")
+
+  -- nil yields nil
   assertEquals(math.tointeger(nil),  nil, "tointeger(nil)")
 
   -- inf and nan yields nil