diff --git a/clang/include/clang/AST/ExternalASTSource.h b/clang/include/clang/AST/ExternalASTSource.h
--- a/clang/include/clang/AST/ExternalASTSource.h
+++ b/clang/include/clang/AST/ExternalASTSource.h
@@ -375,7 +375,9 @@
     if (isOffset()) {
       assert(Source &&
              "Cannot deserialize a lazy pointer without an AST source");
-      Ptr = reinterpret_cast<uint64_t>((Source->*Get)(Ptr >> 1));
+      if (Source != nullptr) {
+        Ptr = reinterpret_cast<uint64_t>((Source->*Get)(Ptr >> 1));
+      }
     }
     return reinterpret_cast<T*>(Ptr);
   }