diff --git a/string.go b/string.go
index ad389f3..0a41082 100644
--- a/string.go
+++ b/string.go
@@ -93,18 +93,18 @@ func formatHelper(l *State, fs string, argCount int) string {
 			case 'd':
 				n := CheckNumber(l, arg)
 				ArgumentCheck(l, math.Floor(n) == n && -math.Pow(2, 63) <= n && n < math.Pow(2, 63), arg, "number has no integer representation")
-				ni := int(n)
+				ni := int64(n)
 				fmt.Fprintf(&b, f, ni)
 			case 'u': // The fmt package doesn't support %u.
 				f = f[:len(f)-1] + "d"
 				n := CheckNumber(l, arg)
 				ArgumentCheck(l, math.Floor(n) == n && 0.0 <= n && n < math.Pow(2, 64), arg, "not a non-negative number in proper range")
-				ni := uint(n)
+				ni := uint64(n)
 				fmt.Fprintf(&b, f, ni)
 			case 'o', 'x', 'X':
 				n := CheckNumber(l, arg)
 				ArgumentCheck(l, 0.0 <= n && n < math.Pow(2, 64), arg, "not a non-negative number in proper range")
-				ni := uint(n)
+				ni := uint64(n)
 				fmt.Fprintf(&b, f, ni)
 			case 'e', 'E', 'f', 'g', 'G':
 				fmt.Fprintf(&b, f, CheckNumber(l, arg))
diff --git a/string_test.go b/string_test.go
new file mode 100644
index 0000000..67bc1ca
--- /dev/null
+++ b/string_test.go
@@ -0,0 +1,14 @@
+package lua
+
+import "testing"
+
+func TestFormatLargeIntegers(t *testing.T) {
+	testString(t, `
+		assert(string.format("%012d", 34567890121111) == "34567890121111")
+		assert(string.format("%i", -34567890121111) == "-34567890121111")
+		assert(string.format("%u", 34567890121111) == "34567890121111")
+		assert(string.format("%o", 34567890121111) == "767016663730627")
+		assert(string.format("%x", 34567890121111) == "1f7076cfb197")
+		assert(string.format("%X", 34567890121111) == "1F7076CFB197")
+	`)
+}