diff --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
index 6a18af8..aaa59fc 100644
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -3823,7 +3823,6 @@
     const FileEntry *ModMap = M ? Map.getModuleMapFileForUniquing(M) : nullptr;
     // Don't emit module relocation error if we have -fno-validate-pch
     if (!PP.getPreprocessorOpts().DisablePCHValidation && !ModMap) {
-      assert(ImportedBy && "top-level import should be verified");
       if ((ClientLoadCapabilities & ARR_OutOfDate) == 0) {
         if (auto *ASTFE = M ? M->getASTFile() : nullptr) {
           // This module was defined by an imported (explicit) module.
@@ -3832,12 +3831,13 @@
         } else {
           // This module was built with a different module map.
           Diag(diag::err_imported_module_not_found)
-              << F.ModuleName << F.FileName << ImportedBy->FileName
-              << F.ModuleMapPath;
+              << F.ModuleName << F.FileName
+              << (ImportedBy ? ImportedBy->FileName : "") << F.ModuleMapPath
+              << !ImportedBy;
           // In case it was imported by a PCH, there's a chance the user is
           // just missing to include the search path to the directory containing
           // the modulemap.
-          if (ImportedBy->Kind == MK_PCH)
+          if (ImportedBy && ImportedBy->Kind == MK_PCH)
             Diag(diag::note_imported_by_pch_module_not_found)
                 << llvm::sys::path::parent_path(F.ModuleMapPath);
         }
@@ -3851,11 +3851,13 @@
     auto StoredModMap = FileMgr.getFile(F.ModuleMapPath);
     if (!StoredModMap || *StoredModMap != ModMap) {
       assert(ModMap && "found module is missing module map file");
-      assert(ImportedBy && "top-level import should be verified");
+      assert((ImportedBy || F.Kind == MK_ImplicitModule) &&
+             "top-level import should be verified");
+      bool NotImported = F.Kind == MK_ImplicitModule && !ImportedBy;
       if ((ClientLoadCapabilities & ARR_OutOfDate) == 0)
         Diag(diag::err_imported_module_modmap_changed)
-          << F.ModuleName << ImportedBy->FileName
-          << ModMap->getName() << F.ModuleMapPath;
+            << F.ModuleName << (NotImported ? F.FileName : ImportedBy->FileName)
+            << ModMap->getName() << F.ModuleMapPath << NotImported;
       return OutOfDate;
     }