diff --git a/clang/include/clang/Basic/DiagnosticLexKinds.td b/clang/include/clang/Basic/DiagnosticLexKinds.td
--- a/clang/include/clang/Basic/DiagnosticLexKinds.td
+++ b/clang/include/clang/Basic/DiagnosticLexKinds.td
@@ -695,6 +695,9 @@
   "no module named '%0' visible from '%1'">;
 def err_mmap_missing_module_qualified : Error<
   "no module named '%0' in '%1'">;
+def err_mmap_missing_parent_module: Error<
+  "no module named '%0' %select{found|in '%2'}1, "
+  "parent module must be defined before the submodule">;
 def err_mmap_top_level_inferred_submodule : Error<
   "only submodules and framework modules may be inferred with wildcard syntax">;
 def err_mmap_inferred_no_umbrella : Error<
diff --git a/clang/lib/Lex/ModuleMap.cpp b/clang/lib/Lex/ModuleMap.cpp
--- a/clang/lib/Lex/ModuleMap.cpp
+++ b/clang/lib/Lex/ModuleMap.cpp
@@ -1903,18 +1903,16 @@
         continue;
       }
 
-      if (ActiveModule) {
-        Diags.Report(Id[I].second, diag::err_mmap_missing_module_qualified)
-          << Id[I].first
-          << ActiveModule->getTopLevelModule()->getFullModuleName();
-      } else {
-        Diags.Report(Id[I].second, diag::err_mmap_expected_module_name);
-      }
+      Diags.Report(Id[I].second, diag::err_mmap_missing_parent_module)
+          << Id[I].first << (ActiveModule != nullptr)
+          << (ActiveModule
+                  ? ActiveModule->getTopLevelModule()->getFullModuleName()
+                  : "");
       HadError = true;
-      return;
     }
 
-    if (ModuleMapFile != Map.getContainingModuleMapFile(TopLevelModule)) {
+    if (TopLevelModule &&
+        ModuleMapFile != Map.getContainingModuleMapFile(TopLevelModule)) {
       assert(ModuleMapFile != Map.getModuleMapFileForUniquing(TopLevelModule) &&
              "submodule defined in same file as 'module *' that allowed its "
              "top-level module");
diff --git a/clang/test/Modules/diagnostics.modulemap b/clang/test/Modules/diagnostics.modulemap
--- a/clang/test/Modules/diagnostics.modulemap
+++ b/clang/test/Modules/diagnostics.modulemap
@@ -28,3 +28,9 @@
   header "quux.h" { size 1 mtime 2 }
   header "no_attrs.h" {}
 }
+
+// CHECK: diagnostics.modulemap:[[@LINE+1]]:8: error: no module named 'unknown' found, parent module must be defined before the submodule
+module unknown.submodule {}
+module known_top_level {}
+// CHECK: diagnostics.modulemap:[[@LINE+1]]:24: error: no module named 'unknown' in 'known_top_level', parent module must be defined before the submodule
+module known_top_level.unknown.submodule {}