Index: source/Utility/ArchSpec.cpp
===================================================================
--- source/Utility/ArchSpec.cpp
+++ source/Utility/ArchSpec.cpp
@@ -859,7 +859,7 @@
 void ArchSpec::MergeFrom(const ArchSpec &other) {
   if (!TripleVendorWasSpecified() && other.TripleVendorWasSpecified())
     GetTriple().setVendor(other.GetTriple().getVendor());
-  if (!TripleOSWasSpecified() && other.TripleVendorWasSpecified())
+  if (!TripleOSWasSpecified() && other.TripleOSWasSpecified())
     GetTriple().setOS(other.GetTriple().getOS());
   if (GetTriple().getArch() == llvm::Triple::UnknownArch) {
     GetTriple().setArch(other.GetTriple().getArch());
Index: unittests/Utility/ArchSpecTest.cpp
===================================================================
--- unittests/Utility/ArchSpecTest.cpp
+++ unittests/Utility/ArchSpecTest.cpp
@@ -10,6 +10,7 @@
 
 #include "lldb/Utility/ArchSpec.h"
 #include "llvm/BinaryFormat/MachO.h"
+#include "llvm/BinaryFormat/ELF.h"
 
 using namespace lldb;
 using namespace lldb_private;
@@ -174,6 +175,31 @@
     EXPECT_EQ(llvm::Triple::EnvironmentType::Android,
               A.GetTriple().getEnvironment());
   }
+  {
+    ArchSpec A, B;
+    A.SetArchitecture(eArchTypeELF, llvm::ELF::EM_ARM,
+                      LLDB_INVALID_CPUTYPE, llvm::ELF::ELFOSABI_NONE);
+    B.SetArchitecture(eArchTypeELF, llvm::ELF::EM_ARM,
+                      LLDB_INVALID_CPUTYPE, llvm::ELF::ELFOSABI_LINUX);
+
+    EXPECT_TRUE(A.IsValid());
+    EXPECT_TRUE(B.IsValid());
+    
+    EXPECT_EQ(llvm::Triple::ArchType::arm, B.GetTriple().getArch());
+    EXPECT_EQ(llvm::Triple::VendorType::UnknownVendor,
+              B.GetTriple().getVendor());
+    EXPECT_EQ(llvm::Triple::OSType::Linux, B.GetTriple().getOS());
+    EXPECT_EQ(llvm::Triple::EnvironmentType::UnknownEnvironment,
+              B.GetTriple().getEnvironment());
+    
+    A.MergeFrom(B);
+    EXPECT_EQ(llvm::Triple::ArchType::arm, A.GetTriple().getArch());
+    EXPECT_EQ(llvm::Triple::VendorType::UnknownVendor,
+              A.GetTriple().getVendor());
+    EXPECT_EQ(llvm::Triple::OSType::Linux, A.GetTriple().getOS());
+    EXPECT_EQ(llvm::Triple::EnvironmentType::UnknownEnvironment,
+              A.GetTriple().getEnvironment());
+  }
 }
 
 TEST(ArchSpecTest, MergeFromMachOUnknown) {