diff --git a/lib/promscrape/client.go b/lib/promscrape/client.go
index f0d329c9f..565e6915c 100644
--- a/lib/promscrape/client.go
+++ b/lib/promscrape/client.go
@@ -184,7 +184,7 @@ func (c *client) ReadData(dst []byte) ([]byte, error) {
 	deadline := time.Now().Add(c.hc.ReadTimeout)
 	req := fasthttp.AcquireRequest()
 	req.SetRequestURI(c.requestURI)
-	req.SetHost(c.host)
+	req.Header.SetHost(c.host)
 	// The following `Accept` header has been copied from Prometheus sources.
 	// See https://github.com/prometheus/prometheus/blob/f9d21f10ecd2a343a381044f131ea4e46381ce09/scrape/scrape.go#L532 .
 	// This is needed as a workaround for scraping stupid Java-based servers such as Spring Boot.
diff --git a/lib/promscrape/discoveryutils/client.go b/lib/promscrape/discoveryutils/client.go
index 293e39831..a26190ee6 100644
--- a/lib/promscrape/discoveryutils/client.go
+++ b/lib/promscrape/discoveryutils/client.go
@@ -42,6 +42,7 @@ type Client struct {
 
 	apiServer string
 
+	hostPort        string
 	authHeader      string
 	proxyAuthHeader string
 	sendFullURL     bool
@@ -127,6 +128,7 @@ func NewClient(apiServer string, ac *promauth.Config, proxyURL proxy.URL, proxyA
 		hc:              hc,
 		blockingClient:  blockingClient,
 		apiServer:       apiServer,
+		hostPort:        hostPort,
 		authHeader:      authHeader,
 		proxyAuthHeader: proxyAuthHeader,
 		sendFullURL:     sendFullURL,
@@ -183,8 +185,8 @@ func (c *Client) getAPIResponseWithParamsAndClient(client *fasthttp.HostClient,
 		req.SetRequestURIBytes(u.FullURI())
 	} else {
 		req.SetRequestURIBytes(u.RequestURI())
-		req.SetHostBytes(u.Host())
 	}
+	req.Header.SetHost(c.hostPort)
 	req.Header.Set("Accept-Encoding", "gzip")
 	if c.authHeader != "" {
 		req.Header.Set("Authorization", c.authHeader)