diff --git a/lib/Serialization/ASTReader.cpp b/lib/Serialization/ASTReader.cpp
index 930b651f9ef00a315aadeb25dbddb1e8ce9e2dd0..d3721bbe34234e067525cb44f84aa55cf9760637 100644
--- a/lib/Serialization/ASTReader.cpp
+++ b/lib/Serialization/ASTReader.cpp
@@ -6285,6 +6285,7 @@ static Decl *getPredefinedDecl(ASTContext &Context, PredefinedDeclIDs ID) {
   case PREDEF_DECL_BUILTIN_VA_LIST_ID:
     return Context.getBuiltinVaListDecl();
   }
+  llvm_unreachable("PredefinedDeclIDs unknown enum value");
 }
 
 Decl *ASTReader::GetExistingDecl(DeclID ID) {