diff --git a/src/libwolfboot.c b/src/libwolfboot.c
index 7994e3ca..41e3380d 100644
--- a/src/libwolfboot.c
+++ b/src/libwolfboot.c
@@ -2587,6 +2587,13 @@ int RAMFUNCTION ext_flash_encrypt_write(uintptr_t address, const uint8_t *data,
     uint8_t ENCRYPT_CACHE[NVM_CACHE_SIZE] XALIGNED_STACK(32);
 #endif
 
+    /* A zero-length request must not turn into a read-modify-write of the
+     * containing block. */
+    if (len < 0)
+        return -1;
+    if (len == 0)
+        return 0;
+
     row_offset = address & (ENCRYPT_BLOCK_SIZE - 1);
     if (row_offset != 0) {
         row_address = address & ~(ENCRYPT_BLOCK_SIZE - 1);
@@ -2657,6 +2664,20 @@ int RAMFUNCTION ext_flash_encrypt_write(uintptr_t address, const uint8_t *data,
         step -= chunk;
     }
 
+    /* Trailing bytes that do not fill a whole block. "address" is block
+     * aligned here, so merge them into the block that already backs them,
+     * the same way the unaligned head above is handled. */
+    step = sz & (ENCRYPT_BLOCK_SIZE - 1);
+    if (step > 0) {
+        if (ext_flash_read(address, block, ENCRYPT_BLOCK_SIZE)
+                != ENCRYPT_BLOCK_SIZE) {
+            return -1;
+        }
+        XMEMCPY(block, data, step);
+        crypto_encrypt(enc_block, block, ENCRYPT_BLOCK_SIZE);
+        ret = ext_flash_write(address, enc_block, ENCRYPT_BLOCK_SIZE);
+    }
+
     return ret;
 }
 
diff --git a/src/update_flash.c b/src/update_flash.c
index 0a720560..1baf63e6 100644
--- a/src/update_flash.c
+++ b/src/update_flash.c
@@ -323,14 +323,16 @@ static int RAMFUNCTION wolfBoot_copy_sector(struct wolfBoot_image *src,
               if (dst->part == PART_SWAP && SWAP_EXT) {
                   if (ext_flash_read((uintptr_t)(src->hdr) + src_sector_offset +
                                        pos,
-                                 (void *)buffer, FLASHBUFFER_SIZE) < 0) {
+                                 (void *)buffer, FLASHBUFFER_SIZE)
+                          != FLASHBUFFER_SIZE) {
                       ret = -1;
                       goto out;
                   }
               } else {
                   if (ext_flash_check_read((uintptr_t)(src->hdr) +
                                          src_sector_offset + pos,
-                                     (void *)buffer, FLASHBUFFER_SIZE) < 0) {
+                                     (void *)buffer, FLASHBUFFER_SIZE)
+                          != FLASHBUFFER_SIZE) {
                       ret = -1;
                       goto out;
                   }
diff --git a/tools/unit-tests/unit-update-flash.c b/tools/unit-tests/unit-update-flash.c
index cee449cc..4f0978e3 100644
--- a/tools/unit-tests/unit-update-flash.c
+++ b/tools/unit-tests/unit-update-flash.c
@@ -601,6 +601,67 @@ static int add_payload_encrypted(uint8_t part, uint32_t version, uint32_t size,
 }
 #endif
 
+#ifdef EXT_ENCRYPTED
+/* ext_flash_encrypt_write() writes whole ENCRYPT_BLOCK_SIZE blocks. A request
+ * whose length is not a multiple of the block size used to drop the trailing
+ * bytes, and a zero-length request used to rewrite the containing block. */
+START_TEST (test_encrypt_write_keeps_trailing_partial_block)
+{
+    uintptr_t base = (uintptr_t)WOLFBOOT_PARTITION_UPDATE_ADDRESS;
+    int len = (2 * ENCRYPT_BLOCK_SIZE) + 5;
+    uint8_t out[(2 * ENCRYPT_BLOCK_SIZE) + 5];
+    uint8_t in[(2 * ENCRYPT_BLOCK_SIZE) + 5];
+    int i, ret;
+
+    reset_mock_stats();
+    prepare_flash();
+    for (i = 0; i < len; i++)
+        in[i] = (uint8_t)(0x30 + i);
+
+    ext_flash_unlock();
+    ret = ext_flash_encrypt_write(base, in, len);
+    ext_flash_lock();
+    ck_assert_int_ge(ret, 0);
+
+    memset(out, 0, sizeof(out));
+    ck_assert_int_eq(ext_flash_decrypt_read(base, out, len), len);
+    ck_assert_int_eq(memcmp(out, in, len), 0);
+
+    cleanup_flash();
+}
+END_TEST
+
+START_TEST (test_encrypt_write_zero_length_leaves_flash_untouched)
+{
+    uintptr_t base = (uintptr_t)WOLFBOOT_PARTITION_UPDATE_ADDRESS;
+    uint8_t before[ENCRYPT_BLOCK_SIZE];
+    uint8_t after[ENCRYPT_BLOCK_SIZE];
+    uint8_t in[ENCRYPT_BLOCK_SIZE];
+    int i, ret;
+
+    reset_mock_stats();
+    prepare_flash();
+    for (i = 0; i < ENCRYPT_BLOCK_SIZE; i++)
+        in[i] = (uint8_t)(0x70 + i);
+
+    ext_flash_unlock();
+    ck_assert_int_ge(ext_flash_encrypt_write(base, in, ENCRYPT_BLOCK_SIZE), 0);
+    ck_assert_int_eq(ext_flash_read(base, before, ENCRYPT_BLOCK_SIZE),
+        ENCRYPT_BLOCK_SIZE);
+
+    ret = ext_flash_encrypt_write(base, in, 0);
+    ext_flash_lock();
+    ck_assert_int_eq(ret, 0);
+
+    ck_assert_int_eq(ext_flash_read(base, after, ENCRYPT_BLOCK_SIZE),
+        ENCRYPT_BLOCK_SIZE);
+    ck_assert_int_eq(memcmp(before, after, ENCRYPT_BLOCK_SIZE), 0);
+
+    cleanup_flash();
+}
+END_TEST
+#endif /* EXT_ENCRYPTED */
+
 START_TEST (test_empty_panic)
 {
     reset_mock_stats();
@@ -1604,6 +1665,9 @@ Suite *wolfboot_suite(void)
     TCase *fallback_verify = tcase_create("Fallback verify");
 #endif
 #endif
+#ifdef EXT_ENCRYPTED
+    TCase *encrypt_write_bounds = tcase_create("Encrypted write bounds");
+#endif
 
 
 #ifdef UNIT_TEST_FALLBACK_ONLY
@@ -1612,6 +1676,11 @@ Suite *wolfboot_suite(void)
     tcase_add_test(fallback_verify, test_final_swap_propagates_encrypt_key_read_failure);
     tcase_add_test(fallback_verify, test_final_swap_propagates_encrypt_key_persist_failure);
     suite_add_tcase(s, fallback_verify);
+    tcase_add_test(encrypt_write_bounds,
+        test_encrypt_write_keeps_trailing_partial_block);
+    tcase_add_test(encrypt_write_bounds,
+        test_encrypt_write_zero_length_leaves_flash_untouched);
+    suite_add_tcase(s, encrypt_write_bounds);
 #endif
     return s;
 #else