diff --git a/libnfc/drivers/pn532_spi.c b/libnfc/drivers/pn532_spi.c
index 05cb000..5824443 100644
--- a/libnfc/drivers/pn532_spi.c
+++ b/libnfc/drivers/pn532_spi.c
@@ -186,7 +186,7 @@ pn532_connstring_decode(const nfc_connstring connstring, struct pn532_spi_descri
     return 2;
   }
   unsigned long speed;
-  if (sscanf(speed_s, "%lu", &speed) != 1) {
+  if (sscanf(speed_s, "%10lu", &speed) != 1) {
     // speed_s is not a number
     free(cs);
     return 2;