diff --git a/pcsc-relay/src/opicc.c b/pcsc-relay/src/opicc.c
index 732a953..4cd42f1 100644
--- a/pcsc-relay/src/opicc.c
+++ b/pcsc-relay/src/opicc.c
@@ -61,7 +61,7 @@ int picc_encode_rapdu(const unsigned char *inbuf, size_t inlen,
     *outlen = length;
 
     /* write length of R-APDU */
-    sprintf(p, "%0lX:", inlen);
+    sprintf(p, "%0lX:", (unsigned long) inlen);
 
     /* next points to the next byte to encode */
     next = inbuf;