diff --git a/libfreefare/mifare_desfire_crypto.c b/libfreefare/mifare_desfire_crypto.c
index de2ff16..3d56699 100644
--- a/libfreefare/mifare_desfire_crypto.c
+++ b/libfreefare/mifare_desfire_crypto.c
@@ -69,6 +69,9 @@ xor(const uint8_t *ivect, uint8_t *data, const size_t len)
 void
 rol(uint8_t *data, const size_t len)
 {
+    if (len == 0)
+        return;
+    
     uint8_t first = data[0];
     for (size_t i = 0; i < len - 1; i++) {
 	data[i] = data[i + 1];
@@ -79,6 +82,9 @@ rol(uint8_t *data, const size_t len)
 void
 lsl(uint8_t *data, size_t len)
 {
+    if (len == 0)
+        return;
+    
     for (size_t n = 0; n < len - 1; n++) {
 	data[n] = (data[n] << 1) | (data[n + 1] >> 7);
     }