diff --git a/.clang-format b/.clang-format
index 6de1483..8f9cb6c 100644
--- a/.clang-format
+++ b/.clang-format
@@ -10,6 +10,7 @@
 IncludeBlocks: Preserve
 TypenameMacros: ['LHASH_OF', 'STACK_OF']
 StatementMacros:
+  - "ASN1_SEQUENCE_END"
   - "DECLARE_ASN1_ALLOC_FUNCTIONS"
   - "DECLARE_ASN1_ALLOC_FUNCTIONS_name"
   - "DECLARE_ASN1_ENCODE_FUNCTIONS"
diff --git a/crypto/asn1/a_object.c b/crypto/asn1/a_object.c
index 91ebb7a..b88f06b 100644
--- a/crypto/asn1/a_object.c
+++ b/crypto/asn1/a_object.c
@@ -70,13 +70,8 @@
 int i2d_ASN1_OBJECT(const ASN1_OBJECT *a, unsigned char **pp)
 {
     if (a == NULL) {
-        /* Match |ASN1_item_i2d| and "successfully" omit the object when passed
-         * NULL.
-         *
-         * TODO(davidben): Should this and |ASN1_item_i2d| be an error? OpenSSL
-         * did not do this, but our |CBB|-based functions like
-         * |i2d_RSAPrivateKey| don't allow NULL at all. */
-        return 0;
+        OPENSSL_PUT_ERROR(ASN1, ERR_R_PASSED_NULL_PARAMETER);
+        return -1;
     }
 
     if (a->length == 0) {
diff --git a/crypto/asn1/asn1_test.cc b/crypto/asn1/asn1_test.cc
index 5cd17af..9119dea 100644
--- a/crypto/asn1/asn1_test.cc
+++ b/crypto/asn1/asn1_test.cc
@@ -1131,7 +1131,7 @@
 DECLARE_ASN1_FUNCTIONS(ASN1_LINKED_LIST)
 
 ASN1_SEQUENCE(ASN1_LINKED_LIST) = {
-  ASN1_OPT(ASN1_LINKED_LIST, next, ASN1_LINKED_LIST),
+    ASN1_OPT(ASN1_LINKED_LIST, next, ASN1_LINKED_LIST),
 } ASN1_SEQUENCE_END(ASN1_LINKED_LIST)
 
 IMPLEMENT_ASN1_FUNCTIONS(ASN1_LINKED_LIST)
@@ -1182,15 +1182,13 @@
   ASN1_STRING *string;
 };
 
-// clang-format off
 DECLARE_ASN1_FUNCTIONS(IMPLICIT_CHOICE)
 
 ASN1_SEQUENCE(IMPLICIT_CHOICE) = {
-  ASN1_IMP(IMPLICIT_CHOICE, string, DIRECTORYSTRING, 0)
+    ASN1_IMP(IMPLICIT_CHOICE, string, DIRECTORYSTRING, 0),
 } ASN1_SEQUENCE_END(IMPLICIT_CHOICE)
 
 IMPLEMENT_ASN1_FUNCTIONS(IMPLICIT_CHOICE)
-// clang-format on
 
 // Test that the ASN.1 templates reject types with implicitly-tagged CHOICE
 // types.
@@ -1215,4 +1213,50 @@
   EXPECT_EQ(nullptr, d2i_IMPLICIT_CHOICE(nullptr, &ptr, sizeof(kInput2)));
 }
 
+struct REQUIRED_FIELD {
+  ASN1_INTEGER *value;
+  ASN1_INTEGER *value_imp;
+  ASN1_INTEGER *value_exp;
+  STACK_OF(ASN1_INTEGER) *seq;
+  STACK_OF(ASN1_INTEGER) *seq_imp;
+  STACK_OF(ASN1_INTEGER) *seq_exp;
+};
+
+DECLARE_ASN1_FUNCTIONS(REQUIRED_FIELD)
+ASN1_SEQUENCE(REQUIRED_FIELD) = {
+    ASN1_SIMPLE(REQUIRED_FIELD, value, ASN1_INTEGER),
+    ASN1_IMP(REQUIRED_FIELD, value_imp, ASN1_INTEGER, 0),
+    ASN1_EXP(REQUIRED_FIELD, value_exp, ASN1_INTEGER, 1),
+    ASN1_SEQUENCE_OF(REQUIRED_FIELD, seq, ASN1_INTEGER),
+    ASN1_IMP_SEQUENCE_OF(REQUIRED_FIELD, seq_imp, ASN1_INTEGER, 2),
+    ASN1_EXP_SEQUENCE_OF(REQUIRED_FIELD, seq_exp, ASN1_INTEGER, 3),
+} ASN1_SEQUENCE_END(REQUIRED_FIELD)
+IMPLEMENT_ASN1_FUNCTIONS(REQUIRED_FIELD)
+
+// Test that structures with missing required fields cannot be serialized. Test
+// the full combination of tagging and SEQUENCE OF.
+TEST(ASN1Test, MissingRequiredField) {
+  EXPECT_EQ(-1, i2d_REQUIRED_FIELD(nullptr, nullptr));
+
+  std::unique_ptr<REQUIRED_FIELD, decltype(&REQUIRED_FIELD_free)> obj(
+      nullptr, REQUIRED_FIELD_free);
+  for (auto field : {&REQUIRED_FIELD::value, &REQUIRED_FIELD::value_imp,
+                     &REQUIRED_FIELD::value_exp}) {
+    obj.reset(REQUIRED_FIELD_new());
+    ASSERT_TRUE(obj);
+    ASN1_INTEGER_free((*obj).*field);
+    (*obj).*field = nullptr;
+    EXPECT_EQ(-1, i2d_REQUIRED_FIELD(obj.get(), nullptr));
+  }
+
+  for (auto field : {&REQUIRED_FIELD::seq, &REQUIRED_FIELD::seq_imp,
+                     &REQUIRED_FIELD::seq_exp}) {
+    obj.reset(REQUIRED_FIELD_new());
+    ASSERT_TRUE(obj);
+    sk_ASN1_INTEGER_pop_free((*obj).*field, ASN1_INTEGER_free);
+    (*obj).*field = nullptr;
+    EXPECT_EQ(-1, i2d_REQUIRED_FIELD(obj.get(), nullptr));
+  }
+}
+
 #endif  // !WINDOWS || !SHARED_LIBRARY
diff --git a/crypto/asn1/internal.h b/crypto/asn1/internal.h
index b995ede..e30be16 100644
--- a/crypto/asn1/internal.h
+++ b/crypto/asn1/internal.h
@@ -124,10 +124,11 @@
                      ASN1_TLC *ctx);
 
 /* ASN1_item_ex_i2d encodes |*pval| as a value of type |it| to |out| under the
- * i2d output convention. It returns the length on success, -1 on error, and
- * zero if the object was omitted. If |tag| is -1. the tag and class come from
- * |it|. Otherwise, the tag number is |tag| and the class is the
- * |ASN1_TFLG_TAG_CLASS| bits of |aclass|. This is used for implicit tagging.
+ * i2d output convention. It returns a non-zero length on success and -1 on
+ * error. If |tag| is -1. the tag and class come from |it|. Otherwise, the tag
+ * number is |tag| and the class is the |ASN1_TFLG_TAG_CLASS| bits of |aclass|.
+ * This is used for implicit tagging. This function treats a missing value as an
+ * error, not an optional field.
  *
  * TODO(davidben): Historically, |aclass| contained other flags, but we may have
  * removed the last of them. */
diff --git a/crypto/asn1/tasn_enc.c b/crypto/asn1/tasn_enc.c
index 7e79da2..6dddb52 100644
--- a/crypto/asn1/tasn_enc.c
+++ b/crypto/asn1/tasn_enc.c
@@ -66,8 +66,12 @@
 #include "internal.h"
 
 
+static int asn1_item_ex_i2d_opt(ASN1_VALUE **pval, unsigned char **out,
+                                const ASN1_ITEM *it, int tag, int aclass,
+                                int optional);
 static int asn1_i2d_ex_primitive(ASN1_VALUE **pval, unsigned char **out,
-                                 const ASN1_ITEM *it, int tag, int aclass);
+                                 const ASN1_ITEM *it, int tag, int aclass,
+                                 int optional);
 static int asn1_ex_i2c(ASN1_VALUE **pval, unsigned char *cont, int *out_omit,
                        int *putype, const ASN1_ITEM *it);
 static int asn1_set_seq_out(STACK_OF(ASN1_VALUE) *sk, unsigned char **out,
@@ -114,14 +118,31 @@
 int ASN1_item_ex_i2d(ASN1_VALUE **pval, unsigned char **out,
                      const ASN1_ITEM *it, int tag, int aclass)
 {
+    int ret = asn1_item_ex_i2d_opt(pval, out, it, tag, aclass, /*optional=*/0);
+    assert(ret != 0);
+    return ret;
+}
+
+/* asn1_item_ex_i2d_opt behaves like |ASN1_item_ex_i2d| but, if |optional| is
+ * non-zero and |*pval| is omitted, it returns zero and writes no bytes. */
+int asn1_item_ex_i2d_opt(ASN1_VALUE **pval, unsigned char **out,
+                         const ASN1_ITEM *it, int tag, int aclass,
+                         int optional)
+{
     const ASN1_TEMPLATE *tt = NULL;
     int i, seqcontlen, seqlen;
-    const ASN1_EXTERN_FUNCS *ef;
     const ASN1_AUX *aux = it->funcs;
     ASN1_aux_cb *asn1_cb = NULL;
 
-    if ((it->itype != ASN1_ITYPE_PRIMITIVE) && !*pval)
-        return 0;
+    /* All fields are pointers, except for boolean |ASN1_ITYPE_PRIMITIVE|s.
+     * Optional primitives are handled later. */
+    if ((it->itype != ASN1_ITYPE_PRIMITIVE) && !*pval) {
+        if (optional) {
+            return 0;
+        }
+        OPENSSL_PUT_ERROR(ASN1, ASN1_R_MISSING_VALUE);
+        return -1;
+    }
 
     if (aux && aux->asn1_cb)
         asn1_cb = aux->asn1_cb;
@@ -129,10 +150,14 @@
     switch (it->itype) {
 
     case ASN1_ITYPE_PRIMITIVE:
-        if (it->templates)
-            return asn1_template_ex_i2d(pval, out, it->templates,
-                                        tag, aclass);
-        return asn1_i2d_ex_primitive(pval, out, it, tag, aclass);
+        if (it->templates) {
+            if (it->templates->flags & ASN1_TFLG_OPTIONAL) {
+                OPENSSL_PUT_ERROR(ASN1, ASN1_R_BAD_TEMPLATE);
+                return -1;
+            }
+            return asn1_template_ex_i2d(pval, out, it->templates, tag, aclass);
+        }
+        return asn1_i2d_ex_primitive(pval, out, it, tag, aclass, optional);
 
     case ASN1_ITYPE_MSTRING:
         /*
@@ -143,7 +168,7 @@
             OPENSSL_PUT_ERROR(ASN1, ASN1_R_BAD_TEMPLATE);
             return -1;
         }
-        return asn1_i2d_ex_primitive(pval, out, it, -1, aclass);
+        return asn1_i2d_ex_primitive(pval, out, it, -1, aclass, optional);
 
     case ASN1_ITYPE_CHOICE: {
         /*
@@ -162,6 +187,10 @@
             return -1;
         }
         const ASN1_TEMPLATE *chtt = it->templates + i;
+        if (chtt->flags & ASN1_TFLG_OPTIONAL) {
+            OPENSSL_PUT_ERROR(ASN1, ASN1_R_BAD_TEMPLATE);
+            return -1;
+        }
         ASN1_VALUE **pchval = asn1_get_field_ptr(pval, chtt);
         int ret = asn1_template_ex_i2d(pchval, out, chtt, -1, aclass);
         if (ret < 0 ||
@@ -171,10 +200,19 @@
         return ret;
     }
 
-    case ASN1_ITYPE_EXTERN:
+    case ASN1_ITYPE_EXTERN: {
         /* If new style i2d it does all the work */
-        ef = it->funcs;
-        return ef->asn1_ex_i2d(pval, out, it, tag, aclass);
+        const ASN1_EXTERN_FUNCS *ef = it->funcs;
+        int ret = ef->asn1_ex_i2d(pval, out, it, tag, aclass);
+        if (ret == 0) {
+            /* |asn1_ex_i2d| should never return zero. We have already checked
+             * for optional values generically, and |ASN1_ITYPE_EXTERN| fields
+             * must be pointers. */
+            OPENSSL_PUT_ERROR(ASN1, ERR_R_INTERNAL_ERROR);
+            return -1;
+        }
+        return ret;
+    }
 
     case ASN1_ITYPE_SEQUENCE:
         i = asn1_enc_restore(&seqcontlen, out, pval, it);
@@ -236,9 +274,10 @@
     }
 }
 
-/* asn1_template_ex_i2d behaves like |ASN1_item_ex_i2d| but uses an
+/* asn1_template_ex_i2d behaves like |asn1_item_ex_i2d_opt| but uses an
  * |ASN1_TEMPLATE| instead of an |ASN1_ITEM|. An |ASN1_TEMPLATE| wraps an
- * |ASN1_ITEM| with modifiers such as tagging, SEQUENCE or SET, etc. */
+ * |ASN1_ITEM| with modifiers such as tagging, SEQUENCE or SET, etc. Instead of
+ * taking an |optional| parameter, it uses the |ASN1_TFLG_OPTIONAL| flag. */
 static int asn1_template_ex_i2d(ASN1_VALUE **pval, unsigned char **out,
                                 const ASN1_TEMPLATE *tt, int tag, int iclass)
 {
@@ -274,6 +313,8 @@
      */
     iclass &= ~ASN1_TFLG_TAG_CLASS;
 
+    const int optional = (flags & ASN1_TFLG_OPTIONAL) != 0;
+
     /*
      * At this point 'ttag' contains the outer tag to use, 'tclass' is the
      * class and iclass is any flags passed to this function.
@@ -286,8 +327,13 @@
         int skcontlen, sklen;
         ASN1_VALUE *skitem;
 
-        if (!*pval)
-            return 0;
+        if (!*pval) {
+            if (optional) {
+                return 0;
+            }
+            OPENSSL_PUT_ERROR(ASN1, ASN1_R_MISSING_VALUE);
+            return -1;
+        }
 
         if (flags & ASN1_TFLG_SET_OF) {
             isset = 1;
@@ -353,7 +399,8 @@
     if (flags & ASN1_TFLG_EXPTAG) {
         /* EXPLICIT tagging */
         /* Find length of tagged item */
-        i = ASN1_item_ex_i2d(pval, NULL, ASN1_ITEM_ptr(tt->item), -1, iclass);
+        i = asn1_item_ex_i2d_opt(pval, NULL, ASN1_ITEM_ptr(tt->item), -1,
+                                 iclass, optional);
         if (i <= 0)
             return i;
         /* Find length of EXPLICIT tag */
@@ -370,8 +417,8 @@
     }
 
     /* Either normal or IMPLICIT tagging: combine class and flags */
-    return ASN1_item_ex_i2d(pval, out, ASN1_ITEM_ptr(tt->item),
-                            ttag, tclass | iclass);
+    return asn1_item_ex_i2d_opt(pval, out, ASN1_ITEM_ptr(tt->item),
+                                ttag, tclass | iclass, optional);
 
 }
 
@@ -463,7 +510,8 @@
 /* asn1_i2d_ex_primitive behaves like |ASN1_item_ex_i2d| but |item| must be a
  * a PRIMITIVE or MSTRING type that is not an |ASN1_ITEM_TEMPLATE|. */
 static int asn1_i2d_ex_primitive(ASN1_VALUE **pval, unsigned char **out,
-                                 const ASN1_ITEM *it, int tag, int aclass)
+                                 const ASN1_ITEM *it, int tag, int aclass,
+                                 int optional)
 {
     /* Get length of content octets and maybe find out the underlying type. */
     int omit;
@@ -473,7 +521,11 @@
         return -1;
     }
     if (omit) {
-        return 0;
+        if (optional) {
+            return 0;
+        }
+        OPENSSL_PUT_ERROR(ASN1, ASN1_R_MISSING_VALUE);
+        return -1;
     }
 
     /*
diff --git a/crypto/x509/x_x509.c b/crypto/x509/x_x509.c
index 38cceb1..9d350bd 100644
--- a/crypto/x509/x_x509.c
+++ b/crypto/x509/x_x509.c
@@ -289,13 +289,15 @@
         return length;
     }
 
-    tmplen = i2d_X509_CERT_AUX(a->aux, pp);
-    if (tmplen < 0) {
-        if (start != NULL)
-            *pp = start;
-        return tmplen;
+    if (a->aux != NULL) {
+        tmplen = i2d_X509_CERT_AUX(a->aux, pp);
+        if (tmplen < 0) {
+            if (start != NULL)
+                *pp = start;
+            return tmplen;
+        }
+        length += tmplen;
     }
-    length += tmplen;
 
     return length;
 }