diff --git a/src/OrchardCore.Modules/OrchardCore.Workflows/Http/Activities/HttpResponseTask.cs b/src/OrchardCore.Modules/OrchardCore.Workflows/Http/Activities/HttpResponseTask.cs
index 16472d4e8fa..34ed0256239 100644
--- a/src/OrchardCore.Modules/OrchardCore.Workflows/Http/Activities/HttpResponseTask.cs
+++ b/src/OrchardCore.Modules/OrchardCore.Workflows/Http/Activities/HttpResponseTask.cs
@@ -80,7 +80,7 @@ public override async Task<ActivityExecutionResult> ExecuteAsync(WorkflowExecuti
 
             foreach (var header in headers)
             {
-                response.Headers.Add(header);
+                response.Headers.Append(header.Key, header.Value);
             }
 
             if (!String.IsNullOrWhiteSpace(contentType))
diff --git a/src/OrchardCore/OrchardCore/Modules/ModularTenantContainerMiddleware.cs b/src/OrchardCore/OrchardCore/Modules/ModularTenantContainerMiddleware.cs
index 6effb7d5e0d..cf07f1ee9af 100644
--- a/src/OrchardCore/OrchardCore/Modules/ModularTenantContainerMiddleware.cs
+++ b/src/OrchardCore/OrchardCore/Modules/ModularTenantContainerMiddleware.cs
@@ -37,7 +37,7 @@ public async Task Invoke(HttpContext httpContext)
             {
                 if (shellSettings.IsInitializing())
                 {
-                    httpContext.Response.Headers.Add(HeaderNames.RetryAfter, "10");
+                    httpContext.Response.Headers.Append(HeaderNames.RetryAfter, "10");
                     httpContext.Response.StatusCode = StatusCodes.Status503ServiceUnavailable;
                     await httpContext.Response.WriteAsync("The requested tenant is currently initializing.");
                     return;