diff --git a/api/prometheus/v1/api.go b/api/prometheus/v1/api.go
index 549cc8d37..138a33b7d 100644
--- a/api/prometheus/v1/api.go
+++ b/api/prometheus/v1/api.go
@@ -1004,7 +1004,8 @@ func (h *apiClientImpl) Do(ctx context.Context, req *http.Request) (*http.Respon
 
 }
 
-// DoGetFallback will attempt to do the request as-is, and on a 405 it will fallback to a GET request.
+// DoGetFallback will attempt to do the request as-is, and on a 405 or 501 it
+// will fallback to a GET request.
 func (h *apiClientImpl) DoGetFallback(ctx context.Context, u *url.URL, args url.Values) (*http.Response, []byte, Warnings, error) {
 	req, err := http.NewRequest(http.MethodPost, u.String(), strings.NewReader(args.Encode()))
 	if err != nil {
@@ -1013,7 +1014,7 @@ func (h *apiClientImpl) DoGetFallback(ctx context.Context, u *url.URL, args url.
 	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
 
 	resp, body, warnings, err := h.Do(ctx, req)
-	if resp != nil && resp.StatusCode == http.StatusMethodNotAllowed {
+	if resp != nil && (resp.StatusCode == http.StatusMethodNotAllowed || resp.StatusCode == http.StatusNotImplemented) {
 		u.RawQuery = args.Encode()
 		req, err = http.NewRequest(http.MethodGet, u.String(), nil)
 		if err != nil {
diff --git a/api/prometheus/v1/api_test.go b/api/prometheus/v1/api_test.go
index 210757504..a85a2ab38 100644
--- a/api/prometheus/v1/api_test.go
+++ b/api/prometheus/v1/api_test.go
@@ -1447,12 +1447,19 @@ func TestDoGetFallback(t *testing.T) {
 		body, _ := json.Marshal(apiResp)
 
 		if req.Method == http.MethodPost {
-			if req.URL.Path == "/blockPost" {
+			if req.URL.Path == "/blockPost405" {
 				http.Error(w, string(body), http.StatusMethodNotAllowed)
 				return
 			}
 		}
 
+		if req.Method == http.MethodPost {
+			if req.URL.Path == "/blockPost501" {
+				http.Error(w, string(body), http.StatusNotImplemented)
+				return
+			}
+		}
+
 		w.Write(body)
 	}))
 	// Close the server when test finishes.
@@ -1483,8 +1490,24 @@ func TestDoGetFallback(t *testing.T) {
 		t.Fatalf("Mismatch in values")
 	}
 
-	// Do a fallbcak to a get.
-	u.Path = "/blockPost"
+	// Do a fallback to a get on 405.
+	u.Path = "/blockPost405"
+	_, b, _, err = api.DoGetFallback(context.TODO(), u, v)
+	if err != nil {
+		t.Fatalf("Error doing local request: %v", err)
+	}
+	if err := json.Unmarshal(b, resp); err != nil {
+		t.Fatal(err)
+	}
+	if resp.Method != http.MethodGet {
+		t.Fatalf("Mismatch method")
+	}
+	if resp.Values != v.Encode() {
+		t.Fatalf("Mismatch in values")
+	}
+
+	// Do a fallback to a get on 501.
+	u.Path = "/blockPost501"
 	_, b, _, err = api.DoGetFallback(context.TODO(), u, v)
 	if err != nil {
 		t.Fatalf("Error doing local request: %v", err)