diff --git a/README.md b/README.md
index 48ab1f3..06910ec 100644
--- a/README.md
+++ b/README.md
@@ -47,14 +47,15 @@ go get github.com/ionos-cloud/sdk-go/v6@latest
 
 ## Environment Variables
 
-| Environment Variable | Description                                                                                                                                                                                                                    |
-|----------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
-| `IONOS_USERNAME`     | Specify the username used to login, to authenticate against the IONOS Cloud API                                                                                                                                                |
-| `IONOS_PASSWORD`     | Specify the password used to login, to authenticate against the IONOS Cloud API                                                                                                                                                |
-| `IONOS_TOKEN`        | Specify the token used to login, if a token is being used instead of username and password                                                                                                                                     |
-| `IONOS_API_URL`      | Specify the API URL. It will overwrite the API endpoint default value `api.ionos.com`. Note: the host URL does not contain the `/cloudapi/v6` path, so it should _not_ be included in the `IONOS_API_URL` environment variable |
-| `IONOS_LOG_LEVEL`     | Specify the Log Level used to log messages. Possible values: Off, Debug, Trace |
-| `IONOS_PINNED_CERT`  | Specify the SHA-256 public fingerprint here, enables certificate pinning                                                                                                                                                       |
+| Environment Variable    | Description                                                                                                                                                                                                                    |
+|-------------------------|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+| `IONOS_USERNAME`        | Specify the username used to login, to authenticate against the IONOS Cloud API                                                                                                                                                |
+| `IONOS_PASSWORD`        | Specify the password used to login, to authenticate against the IONOS Cloud API                                                                                                                                                |
+| `IONOS_TOKEN`           | Specify the token used to login, if a token is being used instead of username and password                                                                                                                                     |
+| `IONOS_API_URL`         | Specify the API URL. It will overwrite the API endpoint default value `api.ionos.com`. Note: the host URL does not contain the `/cloudapi/v6` path, so it should _not_ be included in the `IONOS_API_URL` environment variable |
+| `IONOS_LOG_LEVEL`       | Specify the Log Level used to log messages. Possible values: Off, Debug, Trace                                                                                                                                                 |
+| `IONOS_PINNED_CERT`     | Specify the SHA-256 public fingerprint here, enables certificate pinning                                                                                                                                                       |
+| `IONOS_CONTRACT_NUMBER` | Specify the contract number on which you wish to provision. Only valid for reseller accounts, for other types of accounts the header will be ignored                                                                           |
 
 ⚠️ **_Note: To overwrite the api endpoint - `api.ionos.com`, the environment variable `$IONOS_API_URL` can be set, and used with `NewConfigurationFromEnv()` function._**
 
diff --git a/client.go b/client.go
index 4b5a7e0..d237278 100644
--- a/client.go
+++ b/client.go
@@ -53,7 +53,7 @@ const (
 	RequestStatusFailed  = "FAILED"
 	RequestStatusDone    = "DONE"
 
-	Version = "6.1.8"
+	Version = "6.1.9"
 )
 
 // Constants for APIs
@@ -444,6 +444,15 @@ func (c *APIClient) prepareRequest(
 
 	var body *bytes.Buffer
 
+	val, isSetInEnv := os.LookupEnv(IonosContractNumber)
+	_, isSetInMap := headerParams["X-Contract-Number"]
+	if headerParams == nil {
+		headerParams = make(map[string]string)
+	}
+	if !isSetInMap && isSetInEnv {
+		headerParams["X-Contract-Number"] = val
+	}
+
 	// Detect postBody type and post.
 	if postBody != nil {
 		contentType := headerParams["Content-Type"]
@@ -949,7 +958,8 @@ func (c *APIClient) WaitForRequest(ctx context.Context, path string) (*APIRespon
 		}
 
 		if resp.StatusCode != http.StatusOK {
-			return localVarAPIResponse, fmt.Errorf("WaitForRequest failed; received status code %d from API", resp.StatusCode)
+			msg := fmt.Sprintf("WaitForRequest failed; received status code %d from API", resp.StatusCode)
+			return localVarAPIResponse, NewGenericOpenAPIError(msg, localVarBody, nil, resp.StatusCode)
 		}
 		if status.Metadata != nil && status.Metadata.Status != nil {
 			switch *status.Metadata.Status {
@@ -964,9 +974,7 @@ func (c *APIClient) WaitForRequest(ctx context.Context, path string) (*APIRespon
 				if status.Metadata.Message != nil {
 					message = *status.Metadata.Message
 				}
-				return localVarAPIResponse, errors.New(
-					fmt.Sprintf("Request %s failed: %s", id, message),
-				)
+				return localVarAPIResponse, fmt.Errorf("Request %s failed: %s", id, message)
 			}
 		}
 		select {
@@ -1106,7 +1114,7 @@ func strlen(s string) int {
 	return utf8.RuneCountInString(s)
 }
 
-// GenericOpenAPIError Provides access to the body, error and model on returned errors.
+// GenericOpenAPIError provides access to the body, error and model on returned errors.
 type GenericOpenAPIError struct {
 	statusCode int
 	body       []byte
diff --git a/configuration.go b/configuration.go
index 6845f4a..075fba8 100644
--- a/configuration.go
+++ b/configuration.go
@@ -28,6 +28,7 @@ const (
 	IonosApiUrlEnvVar     = "IONOS_API_URL"
 	IonosPinnedCertEnvVar = "IONOS_PINNED_CERT"
 	IonosLogLevelEnvVar   = "IONOS_LOG_LEVEL"
+	IonosContractNumber   = "IONOS_CONTRACT_NUMBER"
 	DefaultIonosServerUrl = "https://api.ionos.com/cloudapi/v6"
 	DefaultIonosBasePath  = "/cloudapi/v6"
 	defaultMaxRetries     = 3
@@ -130,7 +131,7 @@ func NewConfiguration(username, password, token, hostUrl string) *Configuration
 	cfg := &Configuration{
 		DefaultHeader:      make(map[string]string),
 		DefaultQueryParams: url.Values{},
-		UserAgent:          "ionos-cloud-sdk-go/v6.1.8",
+		UserAgent:          "ionos-cloud-sdk-go/v6.1.9",
 		Debug:              false,
 		Username:           username,
 		Password:           password,