diff --git a/src/api/api_lib.c b/src/api/api_lib.c
index 3bd6be24..d832db4f 100644
--- a/src/api/api_lib.c
+++ b/src/api/api_lib.c
@@ -82,7 +82,7 @@ netconn_new_with_proto_and_callback(enum netconn_type t, u8_t proto,
       LWIP_ASSERT("freeing conn without freeing pcb", conn->pcb.tcp == NULL);
       LWIP_ASSERT("conn has no mbox", conn->mbox != SYS_MBOX_NULL);
       LWIP_ASSERT("conn has no recvmbox", conn->recvmbox != SYS_MBOX_NULL);
-      LWIP_ASSERT("conn->acceptmbox shouldn't exist", conn->acceptmbox != SYS_MBOX_NULL);
+      LWIP_ASSERT("conn->acceptmbox shouldn't exist", conn->acceptmbox == SYS_MBOX_NULL);
       sys_mbox_free(conn->mbox);
       sys_mbox_free(conn->recvmbox);
       memp_free(MEMP_NETCONN, conn);