diff --git a/gui-agent/qubes-gui-runuser.c b/gui-agent/qubes-gui-runuser.c
index bdaced45..006efcd7 100644
--- a/gui-agent/qubes-gui-runuser.c
+++ b/gui-agent/qubes-gui-runuser.c
@@ -45,7 +45,7 @@ static int pam_conv_callback(int num_msg, const struct pam_message **msg,
 {
     int i;
     struct pam_response *resp_array =
-        calloc(sizeof(struct pam_response), num_msg);
+        calloc(num_msg, sizeof(struct pam_response));
 
     if (resp_array == NULL)
         return PAM_BUF_ERR;