diff --git a/src/libopensc/card-default.c b/src/libopensc/card-default.c
index 238e20a0..faee0b21 100644
--- a/src/libopensc/card-default.c
+++ b/src/libopensc/card-default.c
@@ -50,6 +50,7 @@ static int autodetect_class(struct sc_card *card)
 	for (i = 0; i < class_count; i++) {
 		if (card->ctx->debug >= 2)
 			sc_debug(card->ctx, "trying with 0x%02X\n", classes[i]);
+		memset(&apdu, 0, sizeof(apdu));
 		apdu.cla = classes[i];
 		apdu.cse = SC_APDU_CASE_2_SHORT;
 		apdu.ins = 0xC0;