Index: llvm/lib/ObjectYAML/ELFYAML.cpp
===================================================================
--- llvm/lib/ObjectYAML/ELFYAML.cpp
+++ llvm/lib/ObjectYAML/ELFYAML.cpp
@@ -434,10 +434,8 @@
     BCase(EF_AMDGPU_XNACK);
     BCase(EF_AMDGPU_SRAM_ECC);
     break;
-  case ELF::EM_X86_64:
-    break;
   default:
-    llvm_unreachable("Unsupported architecture");
+    break;
   }
 #undef BCase
 #undef BCaseMask
Index: llvm/test/tools/yaml2obj/ELF/eflags.yaml
===================================================================
--- /dev/null
+++ llvm/test/tools/yaml2obj/ELF/eflags.yaml
@@ -0,0 +1,16 @@
+## Check how the 'Flags' key can be used to encode e_flags field values.
+
+## Check we are able to produce no flags for EM_NONE. EM_NONE is an arbitrary
+## e_machine type that has no EF_* values defined for it.
+# RUN: yaml2obj %s -o %t-no-flags
+# RUN: llvm-readelf --file-headers %t-no-flags | FileCheck %s --check-prefix=NOFLAGS
+
+# NOFLAGS: Flags: 0x0{{$}}
+
+--- !ELF
+FileHeader:
+  Class:   ELFCLASS32
+  Data:    ELFDATA2LSB
+  Type:    ET_REL
+  Machine: EM_NONE
+  Flags:   [ ]