diff --git a/test/unit/tcp/test_tcp.c b/test/unit/tcp/test_tcp.c
index 3560879a..42324d47 100644
--- a/test/unit/tcp/test_tcp.c
+++ b/test/unit/tcp/test_tcp.c
@@ -665,6 +665,7 @@ START_TEST(test_tcp_fast_rexmit_wraparound)
   check_seqnos(pcb->unsent, 6, seqnos);
   EXPECT(pcb->unacked == NULL);
   err = tcp_output(pcb);
+  EXPECT_RET(err == ERR_OK);
   EXPECT(txcounters.num_tx_calls == 2);
   EXPECT(txcounters.num_tx_bytes == 2 * (TCP_MSS + 40U));
   memset(&txcounters, 0, sizeof(txcounters));
@@ -753,6 +754,7 @@ START_TEST(test_tcp_rto_rexmit_wraparound)
   check_seqnos(pcb->unsent, 6, seqnos);
   EXPECT(pcb->unacked == NULL);
   err = tcp_output(pcb);
+  EXPECT_RET(err == ERR_OK);
   EXPECT(txcounters.num_tx_calls == 2);
   EXPECT(txcounters.num_tx_bytes == 2 * (TCP_MSS + 40U));
   memset(&txcounters, 0, sizeof(txcounters));
@@ -1101,6 +1103,7 @@ START_TEST(test_tcp_rto_tracking)
   check_seqnos(pcb->unsent, 5, seqnos);
   EXPECT(pcb->unacked == NULL);
   err = tcp_output(pcb);
+  EXPECT_RET(err == ERR_OK);
   EXPECT(txcounters.num_tx_calls == 5);
   EXPECT(txcounters.num_tx_bytes == 5 * (TCP_MSS + 40U));
   memset(&txcounters, 0, sizeof(txcounters));
@@ -1216,6 +1219,7 @@ static void test_tcp_rto_timeout_impl(int link_down)
   err = tcp_write(pcb, &tx_data[0], TCP_MSS, TCP_WRITE_FLAG_COPY);
   EXPECT_RET(err == ERR_OK);
   err = tcp_output(pcb);
+  EXPECT_RET(err == ERR_OK);
   EXPECT(txcounters.num_tx_calls == 1);
   EXPECT(txcounters.num_tx_bytes == 1 * (TCP_MSS + 40U));
   memset(&txcounters, 0, sizeof(txcounters));