diff --git a/ulid.go b/ulid.go
index 73eb6b8..40a0bec 100644
--- a/ulid.go
+++ b/ulid.go
@@ -362,33 +362,33 @@ func (id ULID) MarshalTextTo(dst []byte) error {
 
 // Byte to index table for O(1) lookups when unmarshaling.
 // We use 0xFF as sentinel value for invalid indexes.
-var dec = [...]byte{
-	0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
-	0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
-	0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
-	0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
-	0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0x00, 0x01,
-	0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0xFF, 0xFF,
-	0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0x0A, 0x0B, 0x0C, 0x0D, 0x0E,
-	0x0F, 0x10, 0x11, 0xFF, 0x12, 0x13, 0xFF, 0x14, 0x15, 0xFF,
-	0x16, 0x17, 0x18, 0x19, 0x1A, 0xFF, 0x1B, 0x1C, 0x1D, 0x1E,
-	0x1F, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0x0A, 0x0B, 0x0C,
-	0x0D, 0x0E, 0x0F, 0x10, 0x11, 0xFF, 0x12, 0x13, 0xFF, 0x14,
-	0x15, 0xFF, 0x16, 0x17, 0x18, 0x19, 0x1A, 0xFF, 0x1B, 0x1C,
-	0x1D, 0x1E, 0x1F, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
-	0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
-	0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
-	0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
-	0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
-	0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
-	0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
-	0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
-	0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
-	0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
-	0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
-	0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
-	0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
-	0xFF, 0xFF, 0xFF, 0xFF, 0xFF, 0xFF,
+var dec = [...]int8{
+	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+	-1, -1, -1, -1, -1, -1, -1, -1, 0x00, 0x01,
+	0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, -1, -1,
+	-1, -1, -1, -1, -1, 0x0A, 0x0B, 0x0C, 0x0D, 0x0E,
+	0x0F, 0x10, 0x11, -1, 0x12, 0x13, -1, 0x14, 0x15, -1,
+	0x16, 0x17, 0x18, 0x19, 0x1A, -1, 0x1B, 0x1C, 0x1D, 0x1E,
+	0x1F, -1, -1, -1, -1, -1, -1, 0x0A, 0x0B, 0x0C,
+	0x0D, 0x0E, 0x0F, 0x10, 0x11, -1, 0x12, 0x13, -1, 0x14,
+	0x15, -1, 0x16, 0x17, 0x18, 0x19, 0x1A, -1, 0x1B, 0x1C,
+	0x1D, 0x1E, 0x1F, -1, -1, -1, -1, -1, -1, -1,
+	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+	-1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
+	-1, -1, -1, -1, -1, -1,
 }
 
 // EncodedSize is the length of a text encoded ULID.