diff --git a/clang/lib/Serialization/ModuleManager.cpp b/clang/lib/Serialization/ModuleManager.cpp
index ba78c9ef5af67..4ecb776513d6f 100644
--- a/clang/lib/Serialization/ModuleManager.cpp
+++ b/clang/lib/Serialization/ModuleManager.cpp
@@ -119,7 +119,7 @@ ModuleManager::addModule(StringRef FileName, ModuleKind Type,
   // Note: ExpectedSize and ExpectedModTime will be 0 for MK_ImplicitModule
   // when using an ASTFileSignature.
   if (lookupModuleFile(FileName, ExpectedSize, ExpectedModTime, Entry)) {
-    ErrorStr = "module file out of date";
+    ErrorStr = "module file has a different size or mtime than expected";
     return OutOfDate;
   }
 
diff --git a/clang/test/Modules/explicit-build.cpp b/clang/test/Modules/explicit-build.cpp
index 7fca0082ea28a..fb65508ccf091 100644
--- a/clang/test/Modules/explicit-build.cpp
+++ b/clang/test/Modules/explicit-build.cpp
@@ -199,6 +199,6 @@
 // RUN:            -fmodule-file=%t/c.pcm \
 // RUN:            %s -DHAVE_A -DHAVE_B -DHAVE_C 2>&1 | FileCheck --check-prefix=CHECK-MISMATCHED-B %s
 //
-// CHECK-MISMATCHED-B:      fatal error: module file '{{.*}}b.pcm' is out of date and needs to be rebuilt: module file out of date
+// CHECK-MISMATCHED-B:      fatal error: module file '{{.*}}b.pcm' is out of date and needs to be rebuilt: module file has a different size or mtime than expected
 // CHECK-MISMATCHED-B-NEXT: note: imported by module 'c'
 // CHECK-MISMATCHED-B-NOT:  note: