diff --git a/src/dawn_native/d3d12/ShaderModuleD3D12.cpp b/src/dawn_native/d3d12/ShaderModuleD3D12.cpp
index 3c34203b5d..96bb4ba87d 100644
--- a/src/dawn_native/d3d12/ShaderModuleD3D12.cpp
+++ b/src/dawn_native/d3d12/ShaderModuleD3D12.cpp
@@ -372,11 +372,14 @@ namespace dawn_native { namespace d3d12 {
         // blob.
         // These strings can be HLSL comments because Tint does not emit HLSL comments.
         // TODO(dawn:549): Replace guards strings with something more secure.
-        ASSERT(hlslSource.find("//") == std::string::npos);
+        constexpr char kStartGuard[] = "// Start shader autogenerated by Dawn.";
+        constexpr char kEndGuard[] = "// End shader autogenerated by Dawn.";
+        ASSERT(hlslSource.find(kStartGuard) == std::string::npos);
+        ASSERT(hlslSource.find(kEndGuard) == std::string::npos);
 
-        stream << "// Start shader autogenerated by Dawn.";
+        stream << kStartGuard << "\n";
         stream << hlslSource;
-        stream << "// End of shader autogenerated by Dawn.";
+        stream << "\n" << kEndGuard;
 
         stream << compileFlags;