Index: COFF/Config.h
===================================================================
--- COFF/Config.h
+++ COFF/Config.h
@@ -28,6 +28,7 @@
 class DefinedRelative;
 class StringChunk;
 class Symbol;
+class InputFile;
 
 // Short aliases.
 static const auto AMD64 = llvm::COFF::IMAGE_FILE_MACHINE_AMD64;
@@ -166,7 +167,7 @@
   std::map<std::string, int> AlignComm;
 
   // Used for /failifmismatch.
-  std::map<StringRef, std::pair<StringRef, std::string>> MustMatch;
+  std::map<StringRef, std::pair<StringRef, InputFile *>> MustMatch;
 
   // Used for /alternatename.
   std::map<StringRef, StringRef> AlternateNames;
Index: COFF/Driver.h
===================================================================
--- COFF/Driver.h
+++ COFF/Driver.h
@@ -180,7 +180,7 @@
 // if value matches previous values for the key.
 // This feature used in the directive section to reject
 // incompatible objects.
-void checkFailIfMismatch(StringRef Arg, StringRef Source);
+void checkFailIfMismatch(StringRef Arg, InputFile *Source);
 
 // Convert Windows resource files (.res files) to a .obj file.
 MemoryBufferRef convertResToCOFF(ArrayRef<MemoryBufferRef> MBs);
Index: COFF/Driver.cpp
===================================================================
--- COFF/Driver.cpp
+++ COFF/Driver.cpp
@@ -314,7 +314,7 @@
       Config->Entry = addUndefined(mangle(Arg->getValue()));
       break;
     case OPT_failifmismatch:
-      checkFailIfMismatch(Arg->getValue(), toString(File));
+      checkFailIfMismatch(Arg->getValue(), File);
       break;
     case OPT_incl:
       addUndefined(Arg->getValue());
@@ -1283,7 +1283,7 @@
 
   // Handle /failifmismatch
   for (auto *Arg : Args.filtered(OPT_failifmismatch))
-    checkFailIfMismatch(Arg->getValue(), "cmd-line");
+    checkFailIfMismatch(Arg->getValue(), nullptr);
 
   // Handle /merge
   for (auto *Arg : Args.filtered(OPT_merge))
Index: COFF/DriverUtils.cpp
===================================================================
--- COFF/DriverUtils.cpp
+++ COFF/DriverUtils.cpp
@@ -698,16 +698,20 @@
 
 // Parses a string in the form of "key=value" and check
 // if value matches previous values for the same key.
-void checkFailIfMismatch(StringRef Arg, StringRef Source) {
+void checkFailIfMismatch(StringRef Arg, InputFile *Source) {
   StringRef K, V;
   std::tie(K, V) = Arg.split('=');
   if (K.empty() || V.empty())
     fatal("/failifmismatch: invalid argument: " + Arg);
-  std::pair<StringRef, StringRef> Existing = Config->MustMatch[K];
+  std::pair<StringRef, InputFile *> Existing = Config->MustMatch[K];
   if (!Existing.first.empty() && V != Existing.first) {
+    StringRef CmdLine = "cmd-line";
+    std::string SourceStr = Source ? toString(Source) : CmdLine;
+    std::string ExistingStr =
+        Existing.second ? toString(Existing.second) : CmdLine;
     fatal("/failifmismatch: mismatch detected for '" + K + "':\n>>> " +
-          Existing.second + " has value " + Existing.first + "\n>>> " +
-          Source + " has value " + V);
+          ExistingStr + " has value " + Existing.first + "\n>>> " +
+          SourceStr + " has value " + V);
   }
   Config->MustMatch[K] = {V, Source};
 }