diff --git a/lib/Frontend/ASTUnit.cpp b/lib/Frontend/ASTUnit.cpp
index 242586a..35a9aa4 100644
--- a/lib/Frontend/ASTUnit.cpp
+++ b/lib/Frontend/ASTUnit.cpp
@@ -1689,7 +1689,21 @@
 }
 
 StringRef ASTUnit::getMainFileName() const {
-  return Invocation->getFrontendOpts().Inputs[0].getFile();
+  if (Invocation && !Invocation->getFrontendOpts().Inputs.empty()) {
+    const FrontendInputFile &Input = Invocation->getFrontendOpts().Inputs[0];
+    if (Input.isFile())
+      return Input.getFile();
+    else
+      return Input.getBuffer()->getBufferIdentifier();
+  }
+
+  if (SourceMgr) {
+    if (const FileEntry *
+          FE = SourceMgr->getFileEntryForID(SourceMgr->getMainFileID()))
+      return FE->getName();
+  }
+
+  return StringRef();
 }
 
 ASTUnit *ASTUnit::create(CompilerInvocation *CI,