diff --git a/src/sock.c b/src/sock.c
index 0351dfc..cedbeb3 100644
--- a/src/sock.c
+++ b/src/sock.c
@@ -183,18 +183,15 @@ int patty_ax25_sock_params_set(patty_ax25_sock *sock,
                                patty_ax25_params *params) {
     if (params->flags & PATTY_AX25_PARAM_CLASSES) {
         if (!(params->classes & PATTY_AX25_PARAM_CLASSES_ABM)) {
-            goto error_invalid;
+            goto error_notsup;
         }
 
-        /*
-         * TODO: Eventually support full duplex
-         */
         if (!(params->classes & PATTY_AX25_PARAM_CLASSES_HALF_DUPLEX)) {
-            goto error_invalid;
+            goto error_notsup;
         }
 
         if (params->classes & PATTY_AX25_PARAM_CLASSES_FULL_DUPLEX) {
-            goto error_invalid;
+            goto error_notsup;
         }
     }
 
@@ -259,6 +256,11 @@ int patty_ax25_sock_params_set(patty_ax25_sock *sock,
 
     return 0;
 
+error_notsup:
+    errno = ENOTSUP;
+
+    return -1;
+
 error_invalid:
     errno = EINVAL;