diff --git a/src/pkg/strconv/itoa.go b/src/pkg/strconv/itoa.go
index a633560..a0a7496 100644
--- a/src/pkg/strconv/itoa.go
+++ b/src/pkg/strconv/itoa.go
@@ -6,12 +6,15 @@
 
 // Uitob64 returns the string representation of i in the given base.
 func Uitob64(u uint64, base uint) string {
+	if base < 2 || 36 < base {
+		panic("invalid base " + Uitoa(base))
+	}
 	if u == 0 {
 		return "0"
 	}
 
 	// Assemble decimal in reverse order.
-	var buf [32]byte
+	var buf [64]byte
 	j := len(buf)
 	b := uint64(base)
 	for u > 0 {
diff --git a/src/pkg/strconv/itoa_test.go b/src/pkg/strconv/itoa_test.go
index e0624b5..039ef44 100644
--- a/src/pkg/strconv/itoa_test.go
+++ b/src/pkg/strconv/itoa_test.go
@@ -50,6 +50,7 @@
 	itob64Test{16, 16, "10"},
 	itob64Test{-0x123456789abcdef, 16, "-123456789abcdef"},
 	itob64Test{1<<63 - 1, 16, "7fffffffffffffff"},
+	itob64Test{1<<63 - 1, 2, "111111111111111111111111111111111111111111111111111111111111111"},
 
 	itob64Test{16, 17, "g"},
 	itob64Test{25, 25, "10"},
@@ -135,6 +136,7 @@
 	uitob64Test{1<<63 + 1, 10, "9223372036854775809"},
 	uitob64Test{1<<64 - 2, 10, "18446744073709551614"},
 	uitob64Test{1<<64 - 1, 10, "18446744073709551615"},
+	uitob64Test{1<<64 - 1, 2, "1111111111111111111111111111111111111111111111111111111111111111"},
 }
 
 func TestUitoa(t *testing.T) {