diff --git a/src/uart_flash.c b/src/uart_flash.c
index 676ea3e2..3623c9c9 100644
--- a/src/uart_flash.c
+++ b/src/uart_flash.c
@@ -118,7 +118,7 @@ int  ext_flash_read(uintptr_t address, uint8_t *data, int len)
     }
     for (i = 0; i < len; i++) {
         if (uart_rx_timeout(&data[i]) != 0)
-            return 0;
+            return -1;
         uart_tx(CMD_ACK);
     }
     return i;
diff --git a/tools/unit-tests/Makefile b/tools/unit-tests/Makefile
index 24b7ccab..62e424b2 100644
--- a/tools/unit-tests/Makefile
+++ b/tools/unit-tests/Makefile
@@ -44,6 +44,7 @@ endif
 
 
 TESTS:=unit-parser unit-fdt unit-extflash unit-string unit-spi-flash unit-aes128 \
+       unit-uart-flash \
        unit-aes256 unit-chacha20 unit-pci unit-mock-state unit-sectorflags \
        unit-max-space \
        unit-image unit-image-rsa unit-nvm unit-nvm-flagshome unit-enc-nvm \
@@ -156,6 +157,9 @@ unit-spi-flash: ../../include/target.h unit-spi-flash.c
 unit-qspi-flash: ../../include/target.h unit-qspi-flash.c
 	gcc -o $@ $^ $(CFLAGS) $(LDFLAGS)
 
+unit-uart-flash: ../../include/target.h unit-uart-flash.c ../../src/uart_flash.c
+	gcc -o $@ unit-uart-flash.c $(CFLAGS) $(LDFLAGS)
+
 unit-tpm-rsa-exp: ../../include/target.h unit-tpm-rsa-exp.c ../../src/string.c
 	gcc -o $@ $^ $(CFLAGS) -I$(WOLFBOOT_LIB_WOLFTPM) -DWOLFBOOT_TPM \
 		-DWOLFTPM_USER_SETTINGS -DWOLFBOOT_TPM_VERIFY -DWOLFBOOT_SIGN_RSA2048 \
diff --git a/tools/unit-tests/unit-uart-flash.c b/tools/unit-tests/unit-uart-flash.c
new file mode 100644
index 00000000..12534b81
--- /dev/null
+++ b/tools/unit-tests/unit-uart-flash.c
@@ -0,0 +1,92 @@
+/* unit-uart-flash.c
+ *
+ * Unit tests for the UART flash transport.
+ */
+
+#define UART_FLASH
+
+#include <check.h>
+#include <stdint.h>
+#include <string.h>
+
+static const uint8_t CMD_ACK = 0x06;
+
+static uint8_t rx_script[16];
+static int rx_script_len;
+static int rx_script_pos;
+static uint8_t tx_log[32];
+static int tx_log_len;
+
+uint32_t wolfBoot_current_firmware_version(void)
+{
+    return 0;
+}
+
+int uart_tx(const uint8_t c)
+{
+    ck_assert_int_lt(tx_log_len, (int)sizeof(tx_log));
+    tx_log[tx_log_len++] = c;
+    return 1;
+}
+
+int uart_rx(uint8_t *c)
+{
+    if (rx_script_pos >= rx_script_len)
+        return 0;
+
+    *c = rx_script[rx_script_pos++];
+    return 1;
+}
+
+#include "../../src/uart_flash.c"
+
+static void reset_uart_script(const uint8_t *script, int len)
+{
+    memset(rx_script, 0, sizeof(rx_script));
+    memcpy(rx_script, script, len);
+    rx_script_len = len;
+    rx_script_pos = 0;
+    memset(tx_log, 0, sizeof(tx_log));
+    tx_log_len = 0;
+}
+
+START_TEST(test_ext_flash_read_timeout_returns_error)
+{
+    uint8_t data[2] = {0};
+    uint8_t script[11];
+    int ret;
+
+    memset(script, CMD_ACK, 10);
+    script[10] = 0x5a;
+    reset_uart_script(script, sizeof(script));
+
+    ret = ext_flash_read(0x12345678, data, sizeof(data));
+
+    ck_assert_int_eq(ret, -1);
+    ck_assert_uint_eq(data[0], 0x5a);
+    ck_assert_uint_eq(data[1], 0x00);
+}
+END_TEST
+
+Suite *wolfboot_suite(void)
+{
+    Suite *s = suite_create("wolfBoot");
+    TCase *uart_flash = tcase_create("UART flash");
+
+    tcase_add_test(uart_flash, test_ext_flash_read_timeout_returns_error);
+    tcase_set_timeout(uart_flash, 20);
+    suite_add_tcase(s, uart_flash);
+
+    return s;
+}
+
+int main(void)
+{
+    int fails;
+    Suite *s = wolfboot_suite();
+    SRunner *sr = srunner_create(s);
+    srunner_run_all(sr, CK_NORMAL);
+    fails = srunner_ntests_failed(sr);
+    srunner_free(sr);
+    return fails;
+}