diff --git a/client/space_lua/stdlib/math.ts b/client/space_lua/stdlib/math.ts
index f83c0d3e..ce659ebd 100644
--- a/client/space_lua/stdlib/math.ts
+++ b/client/space_lua/stdlib/math.ts
@@ -4,7 +4,7 @@ import {
   LuaRuntimeError,
   LuaTable,
 } from "../runtime.ts";
-import { isNegativeZero, isTaggedFloat } from "../numeric.ts";
+import { isNegativeZero, isTaggedFloat, makeLuaFloat } from "../numeric.ts";
 
 // Fast unwrap: avoids function call overhead for the common plain-number case
 function untagNumber(x: any): number {
@@ -117,7 +117,8 @@ export const mathApi = new LuaTable({
   modf: new LuaBuiltinFunction((_sf, x: number) => {
     const xn = untagNumber(x);
     const int = Math.trunc(xn);
-    const frac = xn - int;
+    // Guarantee that the `frac` part is always Lua float
+    const frac = makeLuaFloat(xn - int);
     return new LuaMultiRes([int, frac]);
   }),
 
diff --git a/client/space_lua/stdlib/math_test.lua b/client/space_lua/stdlib/math_test.lua
index c6e4c7f7..921c0ac8 100644
--- a/client/space_lua/stdlib/math_test.lua
+++ b/client/space_lua/stdlib/math_test.lua
@@ -81,12 +81,22 @@ end
 -- modf
 do
   local i, f = math.modf(3.5)
+  assertEquals(math.type(i), "integer", "modf int part is integer")
+  assertEquals(math.type(f), "float",   "modf frac part is float")
   assertEquals(i, 3, "modf(3.5) int")
   assertClose(f, 0.5, 1e-12, "modf(3.5) frac")
 
   local i2, f2 = math.modf(-3.5)
+  assertEquals(math.type(i2), "integer", "modf neg int part is integer")
+  assertEquals(math.type(f2), "float",   "modf neg frac part is float")
   assertEquals(i2, -3, "modf(-3.5) int")
   assertClose(f2, -0.5, 1e-12, "modf(-3.5) frac")
+
+  local i3, f3 = math.modf(4.0)
+  assertEquals(math.type(i3), "integer", "modf whole int part is integer")
+  assertEquals(math.type(f3), "float", "modf whole frac is float")
+  assertEquals(i3, 4, "modf(4.0) int")
+  assertEquals(f3, 0.0, "modf(4.0) frac")
 end
 
 -- sqrt/exp/log/pow