diff --git a/instana/instrumentation/flask/vanilla.py b/instana/instrumentation/flask/vanilla.py
index 30c887aa..6a2dee5c 100644
--- a/instana/instrumentation/flask/vanilla.py
+++ b/instana/instrumentation/flask/vanilla.py
@@ -112,7 +112,10 @@ def handle_user_exception_with_instana(wrapped, instance, argv, kwargs):
 
                 if hasattr(response, 'headers'):
                     tracer.inject(scope.span.context, opentracing.Format.HTTP_HEADERS, response.headers)
-                    response.headers.add('Server-Timing', "intid;desc=%s" % scope.span.context.trace_id)
+                    if hasattr(response.headers, 'add'):
+                        response.headers.add('Server-Timing', "intid;desc=%s" % scope.span.context.trace_id)
+                    elif type(response.headers) is dict or hasattr(response.headers, "__dict__"):
+                        response.headers['Server-Timing'] = "intid;desc=%s" % scope.span.context.trace_id
 
             scope.close()
             flask.g.scope = None
diff --git a/instana/instrumentation/flask/with_blinker.py b/instana/instrumentation/flask/with_blinker.py
index 1a4879f2..5d3e68a3 100644
--- a/instana/instrumentation/flask/with_blinker.py
+++ b/instana/instrumentation/flask/with_blinker.py
@@ -102,13 +102,17 @@ def handle_user_exception_with_instana(wrapped, instance, argv, kwargs):
 
                 if hasattr(response, 'headers'):
                     tracer.inject(scope.span.context, opentracing.Format.HTTP_HEADERS, response.headers)
-                    response.headers.add('Server-Timing', "intid;desc=%s" % scope.span.context.trace_id)
+                    if hasattr(response.headers, 'add'):
+                        response.headers.add('Server-Timing', "intid;desc=%s" % scope.span.context.trace_id)
+                    elif type(response.headers) is dict or hasattr(response.headers, "__dict__"):
+                        response.headers['Server-Timing'] = "intid;desc=%s" % scope.span.context.trace_id
 
             scope.close()
             flask.g.scope = None
-        return response
     except Exception as e:
         logger.debug("handle_user_exception_with_instana:", exc_info=True)
+    finally:
+        return response
 
 
 def teardown_request_with_instana(*argv, **kwargs):