diff --git a/src/libopensc/cardctl.h b/src/libopensc/cardctl.h
index 2f467c1d..cad32219 100644
--- a/src/libopensc/cardctl.h
+++ b/src/libopensc/cardctl.h
@@ -943,7 +943,7 @@ typedef struct sc_cardctl_sc_hsm_init_param {
 	size_t user_pin_len;		/* Length of user PIN */
 	u8 user_pin_retry_counter;	/* Retry counter default value */
 	u8 options[2];				/* Initialization options */
-	char dkek_shares;			/* Number of DKEK shares, 0 for card generated, -1 for none */
+	signed char dkek_shares;	/* Number of DKEK shares, 0 for card generated, -1 for none */
 	char *label;				/* Token label to be set in EF.TokenInfo (2F03) */
 } sc_cardctl_sc_hsm_init_param_t;