diff --git a/include/swift/AST/ImportCache.h b/include/swift/AST/ImportCache.h
index b60d8adf24f..2ac7c200146 100644
--- a/include/swift/AST/ImportCache.h
+++ b/include/swift/AST/ImportCache.h
@@ -53,10 +53,12 @@ class ImportSet final :
   friend TrailingObjects;
   friend ImportCache;
 
-  unsigned NumTopLevelImports;
+  unsigned HasHeaderImportModule : 1;
+  unsigned NumTopLevelImports : 31;
   unsigned NumTransitiveImports;
 
-  ImportSet(ArrayRef<ModuleDecl::ImportedModule> topLevelImports,
+  ImportSet(bool hasHeaderImportModule,
+            ArrayRef<ModuleDecl::ImportedModule> topLevelImports,
             ArrayRef<ModuleDecl::ImportedModule> transitiveImports);
 
   ImportSet(const ImportSet &) = delete;
@@ -74,6 +76,13 @@ public:
     return NumTopLevelImports + NumTransitiveImports;
   }
 
+  /// This is a bit of a hack to make module name lookup work properly.
+  /// If our import set includes the ClangImporter's special header import
+  /// module, we have to check it first, before any other imported module.
+  bool hasHeaderImportModule() const {
+    return HasHeaderImportModule;
+  }
+
   ArrayRef<ModuleDecl::ImportedModule> getTopLevelImports() const {
     return {getTrailingObjects<ModuleDecl::ImportedModule>(),
             NumTopLevelImports};
diff --git a/lib/AST/ImportCache.cpp b/lib/AST/ImportCache.cpp
index 6762472fae4..65292efd2a1 100644
--- a/lib/AST/ImportCache.cpp
+++ b/lib/AST/ImportCache.cpp
@@ -23,9 +23,11 @@
 using namespace swift;
 using namespace namelookup;
 
-ImportSet::ImportSet(ArrayRef<ModuleDecl::ImportedModule> topLevelImports,
+ImportSet::ImportSet(bool hasHeaderImportModule,
+                     ArrayRef<ModuleDecl::ImportedModule> topLevelImports,
                      ArrayRef<ModuleDecl::ImportedModule> transitiveImports)
-  : NumTopLevelImports(topLevelImports.size()),
+  : HasHeaderImportModule(hasHeaderImportModule),
+    NumTopLevelImports(topLevelImports.size()),
     NumTransitiveImports(transitiveImports.size()) {
   auto buffer = getTrailingObjects<ModuleDecl::ImportedModule>();
   std::uninitialized_copy(topLevelImports.begin(), topLevelImports.end(),
@@ -78,6 +80,11 @@ static void collectExports(ModuleDecl::ImportedModule next,
 ImportSet &
 ImportCache::getImportSet(ASTContext &ctx,
                           ArrayRef<ModuleDecl::ImportedModule> imports) {
+  bool hasHeaderImportModule = false;
+  ModuleDecl *headerImportModule = nullptr;
+  if (auto *loader = ctx.getClangModuleLoader())
+    headerImportModule = loader->getImportedHeaderModule();
+
   SmallVector<ModuleDecl::ImportedModule, 4> topLevelImports;
 
   SmallVector<ModuleDecl::ImportedModule, 4> transitiveImports;
@@ -88,6 +95,8 @@ ImportCache::getImportSet(ASTContext &ctx,
       continue;
 
     topLevelImports.push_back(next);
+    if (next.second == headerImportModule)
+      hasHeaderImportModule = true;
   }
 
   void *InsertPos = nullptr;
@@ -116,6 +125,9 @@ ImportCache::getImportSet(ASTContext &ctx,
       continue;
 
     transitiveImports.push_back(next);
+    if (next.second == headerImportModule)
+      hasHeaderImportModule = true;
+
     collectExports(next, stack);
   }
 
@@ -131,7 +143,9 @@ ImportCache::getImportSet(ASTContext &ctx,
     sizeof(ModuleDecl::ImportedModule) * transitiveImports.size(),
     alignof(ImportSet), AllocationArena::Permanent);
 
-  auto *result = new (mem) ImportSet(topLevelImports, transitiveImports);
+  auto *result = new (mem) ImportSet(hasHeaderImportModule,
+                                     topLevelImports,
+                                     transitiveImports);
   ImportSets.InsertNode(result, InsertPos);
 
   return *result;