1 files changed, 3 insertions, 0 deletions
diff --git a/caesar.go b/caesar.go
index 4829e6a..9e36d76 100644
--- a/caesar.go
+++ b/caesar.go
@@ -34,6 +34,9 @@ func (rr RuneRange) size() int {
 // shift shifts r by d within the range, modulo the size of the range.
 func (rr RuneRange) shift(r rune, d int) rune {
 	pos := int(r - rr.Start)
+	for d < 0 {
+		d = rr.size() + d
+	}
 	newPos := (pos + d) % rr.size()
 	return rr.Start + rune(newPos)
 }