diff --git a/pkg/promclient/promclient.go b/pkg/promclient/promclient.go
index 44695a20ce8..f06e9ce54e0 100644
--- a/pkg/promclient/promclient.go
+++ b/pkg/promclient/promclient.go
@@ -105,7 +105,7 @@ func NewWithTracingClient(logger log.Logger, httpClient *http.Client, userAgent
 
 // req2xx sends a request to the given url.URL. If method is http.MethodPost then
 // the raw query is encoded in the body and the appropriate Content-Type is set.
-func (c *Client) req2xx(ctx context.Context, u *url.URL, method string, headers map[string]string) (_ []byte, _ int, err error) {
+func (c *Client) req2xx(ctx context.Context, u *url.URL, method string, headers http.Header) (_ []byte, _ int, err error) {
 	var b io.Reader
 	if method == http.MethodPost {
 		rq := u.RawQuery
@@ -117,6 +117,10 @@ func (c *Client) req2xx(ctx context.Context, u *url.URL, method string, headers
 	if err != nil {
 		return nil, 0, errors.Wrapf(err, "create %s request", method)
 	}
+	if headers != nil {
+		req.Header = headers
+	}
+
 	if c.userAgent != "" {
 		req.Header.Set("User-Agent", c.userAgent)
 	}
@@ -124,10 +128,6 @@ func (c *Client) req2xx(ctx context.Context, u *url.URL, method string, headers
 		req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
 	}
 
-	for header, value := range headers {
-		req.Header.Set(header, value)
-	}
-
 	resp, err := c.Do(req.WithContext(ctx))
 	if err != nil {
 		return nil, 0, errors.Wrapf(err, "perform %s request against %s", method, u.String())
@@ -367,7 +367,7 @@ type QueryOptions struct {
 	MaxSourceResolution     string
 	Engine                  string
 	Explain                 bool
-	HTTPHeaders             map[string]string
+	HTTPHeaders             http.Header
 }
 
 func (p *QueryOptions) AddTo(values url.Values) error {
diff --git a/test/e2e/store_gateway_test.go b/test/e2e/store_gateway_test.go
index 8e90fe0370e..e7ac78f67ed 100644
--- a/test/e2e/store_gateway_test.go
+++ b/test/e2e/store_gateway_test.go
@@ -136,10 +136,13 @@ metafile_content_ttl: 0s`, memcached.InternalEndpoint("memcached"))
 	testutil.Ok(t, s1.WaitSumMetrics(e2emon.Equals(0), "thanos_bucket_store_block_load_failures_total"))
 
 	t.Run("query works", func(t *testing.T) {
+		tenant1Header := make(http.Header)
+		tenant1Header.Add("thanos-tenant", "test-tenant-1")
 		queryAndAssertSeries(t, ctx, q.Endpoint("http"), func() string { return fmt.Sprintf("%s @ end()", testQuery) },
 			time.Now, promclient.QueryOptions{
 				Deduplicate: false,
-				HTTPHeaders: map[string]string{"thanos-tenant": "test-tenant-1"},
+				HTTPHeaders: tenant1Header,
+				// map[string][]string{"thanos-tenant": "test-tenant-1"},
 			},
 			[]model.Metric{
 				{
@@ -173,10 +176,12 @@ metafile_content_ttl: 0s`, memcached.InternalEndpoint("memcached"))
 		testutil.Ok(t, s1.WaitSumMetricsWithOptions(e2emon.Equals(9), []string{"thanos_bucket_store_series_data_fetched"}, e2emon.WithLabelMatchers(matchers.MustNewMatcher(matchers.MatchEqual, tenancy.MetricLabel, "test-tenant-1"))))
 		testutil.Ok(t, s1.WaitSumMetricsWithOptions(e2emon.Equals(3), []string{"thanos_bucket_store_series_blocks_queried"}, e2emon.WithLabelMatchers(matchers.MustNewMatcher(matchers.MatchEqual, tenancy.MetricLabel, "test-tenant-1"))))
 
+		tenant2Header := make(http.Header)
+		tenant2Header.Add("thanos-tenant", "test-tenant-2")
 		queryAndAssertSeries(t, ctx, q.Endpoint("http"), func() string { return testQuery },
 			time.Now, promclient.QueryOptions{
 				Deduplicate: true,
-				HTTPHeaders: map[string]string{"thanos-tenant": "test-tenant-2"},
+				HTTPHeaders: tenant2Header,
 			},
 			[]model.Metric{
 				{