diff --git a/crypto/bytestring/bytestring_test.c b/crypto/bytestring/bytestring_test.c
index fbfe61bf..e02eeaa5 100644
--- a/crypto/bytestring/bytestring_test.c
+++ b/crypto/bytestring/bytestring_test.c
@@ -346,7 +346,8 @@ static int test_cbb_asn1() {
   }
   free(buf);
 
-  test_data = calloc(1, 100000);
+  test_data = malloc(100000);
+  memset(test_data, 0x42, 100000);
 
   if (!CBB_init(&cbb, 0) ||
       !CBB_add_asn1(&cbb, &contents, 0x30) ||
@@ -355,7 +356,9 @@ static int test_cbb_asn1() {
     return 0;
   }
 
-  if (buf_len != 3 + 130 || memcmp(buf, "\x30\x81\x82", 3) != 0) {
+  if (buf_len != 3 + 130 ||
+      memcmp(buf, "\x30\x81\x82", 3) != 0 ||
+      memcmp(buf + 3, test_data, 130) != 0) {
     return 0;
   }
   free(buf);
@@ -367,7 +370,9 @@ static int test_cbb_asn1() {
     return 0;
   }
 
-  if (buf_len != 4 + 1000 || memcmp(buf, "\x30\x82\x03\xe8", 4) != 0) {
+  if (buf_len != 4 + 1000 ||
+      memcmp(buf, "\x30\x82\x03\xe8", 4) != 0 ||
+      memcmp(buf + 4, test_data, 1000)) {
     return 0;
   }
   free(buf);
@@ -381,7 +386,8 @@ static int test_cbb_asn1() {
   }
 
   if (buf_len != 5 + 5 + 100000 ||
-      memcmp(buf, "\x30\x83\x01\x86\xa5\x30\x83\x01\x86\xa0", 10) != 0) {
+      memcmp(buf, "\x30\x83\x01\x86\xa5\x30\x83\x01\x86\xa0", 10) != 0 ||
+      memcmp(buf + 10, test_data, 100000)) {
     return 0;
   }
   free(buf);
diff --git a/crypto/bytestring/cbb.c b/crypto/bytestring/cbb.c
index 3e35d5d5..850fa02b 100644
--- a/crypto/bytestring/cbb.c
+++ b/crypto/bytestring/cbb.c
@@ -14,6 +14,8 @@
 
 #include <openssl/bytestring.h>
 
+#include <assert.h>
+
 #include <openssl/mem.h>
 
 
@@ -174,6 +176,8 @@ int CBB_flush(CBB *cbb) {
     size_t len_len;
     uint8_t initial_length_byte;
 
+    assert (cbb->pending_len_len == 1);
+
     if (len > 0xfffffffe) {
       /* Too large. */
       return 0;
@@ -201,8 +205,8 @@ int CBB_flush(CBB *cbb) {
       if (!cbb_buffer_add(cbb->base, NULL, extra_bytes)) {
         return 0;
       }
-      memmove(cbb->base->buf + cbb->offset + extra_bytes,
-              cbb->base->buf + cbb->offset, len);
+      memmove(cbb->base->buf + child_start + extra_bytes,
+              cbb->base->buf + child_start, len);
     }
     cbb->base->buf[cbb->offset++] = initial_length_byte;
     cbb->pending_len_len = len_len - 1;