diff --git a/ulid.go b/ulid.go
index d7b99d3..73eb6b8 100644
--- a/ulid.go
+++ b/ulid.go
@@ -362,7 +362,7 @@ 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 = [...]int8{
+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,