Index: test/tools/yaml2obj/duplicate-section-names.test
===================================================================
--- test/tools/yaml2obj/duplicate-section-names.test
+++ test/tools/yaml2obj/duplicate-section-names.test
@@ -29,7 +29,7 @@
 ## sections with equal names and suffixes.
 
 # RUN: not yaml2obj --docnum=2 %s 2>&1 | FileCheck %s --check-prefix=CASE2
-# CASE2: error: Repeated section name: '.foo [1]' at YAML section number 1.
+# CASE2: error: Repeated section name: '.foo [1]' at YAML section number 2.
 
 --- !ELF
 FileHeader:
@@ -48,7 +48,7 @@
 ## names are equal.
 
 # RUN: not yaml2obj --docnum=3 %s 2>&1 | FileCheck %s --check-prefix=CASE3
-# CASE3: error: Repeated section name: '.foo' at YAML section number 1.
+# CASE3: error: Repeated section name: '.foo' at YAML section number 2.
 
 --- !ELF
 FileHeader:
Index: tools/yaml2obj/yaml2elf.cpp
===================================================================
--- tools/yaml2obj/yaml2elf.cpp
+++ tools/yaml2obj/yaml2elf.cpp
@@ -192,6 +192,12 @@
     if (!D->Name.empty())
       DocSections.insert(D->Name);
 
+  // Insert SHT_NULL section implicitly.
+  Doc.Sections.insert(
+      Doc.Sections.begin(),
+      llvm::make_unique<ELFYAML::Section>(
+          ELFYAML::Section::SectionKind::RawContent, true /*IsImplicit*/));
+
   std::vector<StringRef> ImplicitSections = {".symtab", ".strtab", ".shstrtab"};
   if (!Doc.DynamicSymbols.empty())
     ImplicitSections.insert(ImplicitSections.end(), {".dynsym", ".dynstr"});
@@ -317,13 +323,11 @@
                                         ContiguousBlobAccumulator &CBA) {
   // Ensure SHN_UNDEF entry is present. An all-zero section header is a
   // valid SHN_UNDEF entry since SHT_NULL == 0.
-  SHeaders.resize(Doc.Sections.size() + 1);
-  zero(SHeaders[0]);
+  SHeaders.resize(Doc.Sections.size());
 
-  for (size_t I = 1; I < Doc.Sections.size() + 1; ++I) {
+  for (size_t I = 1; I < Doc.Sections.size(); ++I) {
     Elf_Shdr &SHeader = SHeaders[I];
-    zero(SHeader);
-    ELFYAML::Section *Sec = Doc.Sections[I - 1].get();
+    ELFYAML::Section *Sec = Doc.Sections[I].get();
 
     // We have a few sections like string or symbol tables that are usually
     // added implicitly to the end. However, if they are explicitly specified
@@ -944,13 +948,12 @@
 }
 
 template <class ELFT> bool ELFState<ELFT>::buildSectionIndex() {
-  for (unsigned i = 0, e = Doc.Sections.size(); i != e; ++i) {
-    StringRef Name = Doc.Sections[i]->Name;
+  for (unsigned I = 1, E = Doc.Sections.size(); I != E; ++I) {
+    StringRef Name = Doc.Sections[I]->Name;
     DotShStrtab.add(dropUniqueSuffix(Name));
-    // "+ 1" to take into account the SHT_NULL entry.
-    if (!SN2I.addName(Name, i + 1)) {
+    if (!SN2I.addName(Name, I)) {
       WithColor::error() << "Repeated section name: '" << Name
-                         << "' at YAML section number " << i << ".\n";
+                         << "' at YAML section number " << I << ".\n";
       return false;
     }
   }