Index: llvm/include/llvm/ObjectYAML/ELFYAML.h
===================================================================
--- llvm/include/llvm/ObjectYAML/ELFYAML.h
+++ llvm/include/llvm/ObjectYAML/ELFYAML.h
@@ -103,7 +103,7 @@
 struct Symbol {
   StringRef Name;
   ELF_STT Type;
-  StringRef Section;
+  Optional<StringRef> Section;
   Optional<ELF_SHN> Index;
   ELF_STB Binding;
   llvm::yaml::Hex64 Value;
Index: llvm/lib/ObjectYAML/ELFEmitter.cpp
===================================================================
--- llvm/lib/ObjectYAML/ELFEmitter.cpp
+++ llvm/lib/ObjectYAML/ELFEmitter.cpp
@@ -832,8 +832,8 @@
       Symbol.st_name = Strtab.getOffset(ELFYAML::dropUniqueSuffix(Sym.Name));
 
     Symbol.setBindingAndType(Sym.Binding, Sym.Type);
-    if (!Sym.Section.empty())
-      Symbol.st_shndx = toSectionIndex(Sym.Section, "", Sym.Name);
+    if (Sym.Section)
+      Symbol.st_shndx = toSectionIndex(*Sym.Section, "", Sym.Name);
     else if (Sym.Index)
       Symbol.st_shndx = *Sym.Index;
 
Index: llvm/lib/ObjectYAML/ELFYAML.cpp
===================================================================
--- llvm/lib/ObjectYAML/ELFYAML.cpp
+++ llvm/lib/ObjectYAML/ELFYAML.cpp
@@ -1086,7 +1086,7 @@
   IO.mapOptional("Name", Symbol.Name, StringRef());
   IO.mapOptional("StName", Symbol.StName);
   IO.mapOptional("Type", Symbol.Type, ELFYAML::ELF_STT(0));
-  IO.mapOptional("Section", Symbol.Section, StringRef());
+  IO.mapOptional("Section", Symbol.Section);
   IO.mapOptional("Index", Symbol.Index);
   IO.mapOptional("Binding", Symbol.Binding, ELFYAML::ELF_STB(0));
   IO.mapOptional("Value", Symbol.Value, Hex64(0));
@@ -1104,7 +1104,7 @@
 
 std::string MappingTraits<ELFYAML::Symbol>::validate(IO &IO,
                                                      ELFYAML::Symbol &Symbol) {
-  if (Symbol.Index && Symbol.Section.data())
+  if (Symbol.Index && Symbol.Section)
     return "Index and Section cannot both be specified for Symbol";
   return "";
 }
Index: llvm/test/tools/yaml2obj/ELF/dynamic-symbols.yaml
===================================================================
--- llvm/test/tools/yaml2obj/ELF/dynamic-symbols.yaml
+++ llvm/test/tools/yaml2obj/ELF/dynamic-symbols.yaml
@@ -42,7 +42,7 @@
 
 ## Check we can use numeric values to refer to sections.
 
-# RUN: yaml2obj --docnum=2 %s -o %t2
+# RUN: yaml2obj --docnum=2 -DSEC=0xff %s -o %t2
 # RUN: llvm-readobj --dyn-symbols %t2 2>&1 | FileCheck -DFILE=%t2 %s --check-prefix=NUM
 
 # NUM:      Name: foo
@@ -53,11 +53,15 @@
 # NUM:      Section:
 # NUM-SAME: .dynsym (0x2)
 
-# NUM:      Name: zed
+# NUM:      Name: zed1
 # NUM: warning: '[[FILE]]': invalid section index: 255
 # NUM:      Section:
 # NUM-SAME: <?> (0xFF)
 
+# NUM:      Name: zed2
+# NUM:      Section:
+# NUM-SAME: Undefined (0x0)
+
 --- !ELF
 FileHeader:
   Class: ELFCLASS64
@@ -67,12 +71,27 @@
   - Name: .data
     Type: SHT_PROGBITS
 DynamicSymbols:
-  - Name: foo
+  - Name:    foo
     Section: 1
-  - Name: bar
+  - Name:    bar
     Section: 2
-  - Name: zed
-    Section: 0xff
+  - Name:    zed1
+    Section: [[SEC=<none>]]
+## The case when no "Section" key is set.
+  - Name: zed2
+
+## Check that by default no section is set.
+
+# RUN: yaml2obj --docnum=2 %s -o %t2.none
+# RUN: llvm-readobj --dyn-symbols %t2.none 2>&1 | \
+# RUN:   FileCheck -DFILE=%t2.none %s --check-prefix=NONE
+
+# NONE:      Name: zed1
+# NONE:      Section:
+# NONE-SAME: Undefined (0x0)
+# NONE:      Name: zed2
+# NONE:      Section:
+# NONE-SAME: Undefined (0x0)
 
 ## Check we report errors when unknown sections are referenced by dynamic symbols.