diff --git a/src/libopensc/card-oberthur.h b/src/libopensc/card-oberthur.h
index fb2bf7f9..6462baae 100644
--- a/src/libopensc/card-oberthur.h
+++ b/src/libopensc/card-oberthur.h
@@ -26,7 +26,7 @@
 struct NTLV {
 	char *name;
 	unsigned int tag;
-	int len;
+	size_t len;
 	unsigned char *value;
 };
 typedef struct NTLV NTLV_t;