diff --git a/clang-tools-extra/clangd/SourceCode.h b/clang-tools-extra/clangd/SourceCode.h
--- a/clang-tools-extra/clangd/SourceCode.h
+++ b/clang-tools-extra/clangd/SourceCode.h
@@ -281,7 +281,8 @@
   llvm::StringRef Name;
   const MacroInfo *Info;
 };
-/// Gets the macro at a specified \p Loc.
+/// Gets the macro at a specified \p Loc. It must be a spelling location and
+/// point to the beginning of identifier.
 llvm::Optional<DefinedMacro> locateMacroAt(SourceLocation Loc,
                                            Preprocessor &PP);
 
diff --git a/clang-tools-extra/clangd/SourceCode.cpp b/clang-tools-extra/clangd/SourceCode.cpp
--- a/clang-tools-extra/clangd/SourceCode.cpp
+++ b/clang-tools-extra/clangd/SourceCode.cpp
@@ -893,10 +893,11 @@
 
 llvm::Optional<DefinedMacro> locateMacroAt(SourceLocation Loc,
                                            Preprocessor &PP) {
+  assert(Loc.isFileID());
   const auto &SM = PP.getSourceManager();
   const auto &LangOpts = PP.getLangOpts();
   Token Result;
-  if (Lexer::getRawToken(SM.getSpellingLoc(Loc), Result, SM, LangOpts, false))
+  if (Lexer::getRawToken(Loc, Result, SM, LangOpts, false))
     return None;
   if (Result.is(tok::raw_identifier))
     PP.LookUpIdentifierInfo(Result);
@@ -904,14 +905,12 @@
   if (!IdentifierInfo || !IdentifierInfo->hadMacroDefinition())
     return None;
 
-  std::pair<FileID, unsigned int> DecLoc = SM.getDecomposedExpansionLoc(Loc);
   // Get the definition just before the searched location so that a macro
-  // referenced in a '#undef MACRO' can still be found.
-  SourceLocation BeforeSearchedLocation =
-      SM.getMacroArgExpandedLocation(SM.getLocForStartOfFile(DecLoc.first)
-                                         .getLocWithOffset(DecLoc.second - 1));
-  MacroDefinition MacroDef =
-      PP.getMacroDefinitionAtLoc(IdentifierInfo, BeforeSearchedLocation);
+  // referenced in a '#undef MACRO' can still be found. Note that we only do
+  // that if Loc is not pointing at start of file.
+  if (SM.getLocForStartOfFile(SM.getFileID(Loc)) != Loc)
+    Loc = Loc.getLocWithOffset(-1);
+  MacroDefinition MacroDef = PP.getMacroDefinitionAtLoc(IdentifierInfo, Loc);
   if (auto *MI = MacroDef.getMacroInfo())
     return DefinedMacro{IdentifierInfo->getName(), MI};
   return None;
diff --git a/clang-tools-extra/clangd/unittests/SourceCodeTests.cpp b/clang-tools-extra/clangd/unittests/SourceCodeTests.cpp
--- a/clang-tools-extra/clangd/unittests/SourceCodeTests.cpp
+++ b/clang-tools-extra/clangd/unittests/SourceCodeTests.cpp
@@ -436,6 +436,20 @@
   EXPECT_THAT(*Result, MacroName("MACRO"));
 }
 
+TEST(SourceCodeTests, WorksAtBeginOfFile) {
+  Annotations Code("^MACRO");
+  TestTU TU = TestTU::withCode(Code.code());
+  TU.HeaderCode = "#define MACRO int x;";
+  auto AST = TU.build();
+  auto CurLoc = sourceLocationInMainFile(AST.getSourceManager(), Code.point());
+  ASSERT_TRUE(bool(CurLoc));
+  const auto *Id = syntax::spelledIdentifierTouching(*CurLoc, AST.getTokens());
+  ASSERT_TRUE(Id);
+  auto Result = locateMacroAt(Id->location(), AST.getPreprocessor());
+  ASSERT_TRUE(Result);
+  EXPECT_THAT(*Result, MacroName("MACRO"));
+}
+
 TEST(SourceCodeTests, IsInsideMainFile){
   TestTU TU;
   TU.HeaderCode = R"cpp(