diff --git a/plugins_tools/eid-viewer/p11.c b/plugins_tools/eid-viewer/p11.c
index d8df0487a..3ed4bebf2 100644
--- a/plugins_tools/eid-viewer/p11.c
+++ b/plugins_tools/eid-viewer/p11.c
@@ -491,7 +491,7 @@ static int eid_vwr_p11_do_challenge_real(struct eid_vwr_challenge_responsedata *
 		//initialize the signature operation
 		check_rv(C_SignInit(session, &mechanism, hKey));
 
-		check_rv(C_Sign(session, p->challenge, (CK_ULONG)p->challengelen, p->response, &(p->responselen)));
+		check_rv(C_Sign(session, p->challenge, (CK_ULONG)p->challengelen, p->response, &((CK_ULONG)(p->responselen))));
 
 		p->result = EID_VWR_RES_SUCCESS;
 	}