diff --git a/src/methods/download.ts b/src/methods/download.ts
index 5e4065a..8cef3ad 100644
--- a/src/methods/download.ts
+++ b/src/methods/download.ts
@@ -88,7 +88,7 @@ export async function getCidUrl(
 ): Promise<string> {
   const opt = { ...this.customOptions, customOptions };
   return addUrlQuery(path.join(this.portalUrl, cid), {
-    auth_token: opt.ApiKey,
+    auth_token: opt.apiKey,
   });
 }
 
diff --git a/src/utils/options.ts b/src/utils/options.ts
index 19bba68..fc2c8e9 100644
--- a/src/utils/options.ts
+++ b/src/utils/options.ts
@@ -1,10 +1,10 @@
-import { AxiosProgressEvent, AxiosRequestConfig } from "axios";
-import { S5Client } from "../client.js";
+import {AxiosHeaders, AxiosProgressEvent, AxiosRequestConfig} from "axios";
+import {S5Client} from "../client.js";
 
 /**
  * Custom client options.
  *
- * @property [ApiKey] - Authentication password to use for a single S5 node/portal.
+ * @property [apiKey] - Authentication password to use for a single S5 node/portal.
  * @property [customUserAgent] - Custom user agent header to set.
  * @property [customCookie] - Custom cookie header to set. WARNING: the Cookie header cannot be set in browsers. This is meant for usage in server contexts.
  * @property [onDownloadProgress] - Optional callback to track download progress.
@@ -12,69 +12,64 @@ import { S5Client } from "../client.js";
  */
 
 export type CustomClientOptions = {
-  ApiKey?: string;
-  customUserAgent?: string;
-  customCookie?: string;
-  onDownloadProgress?: (progressEvent: AxiosProgressEvent) => void;
-  onUploadProgress?: (progressEvent: AxiosProgressEvent) => void;
+    apiKey?: string;
+    customUserAgent?: string;
+    customCookie?: string;
+    onDownloadProgress?: (progressEvent: AxiosProgressEvent) => void;
+    onUploadProgress?: (progressEvent: AxiosProgressEvent) => void;
 };
 
 export function optionsToConfig(
-  client: S5Client,
-  def: CustomClientOptions,
-  ...options: CustomClientOptions[]
+    client: S5Client,
+    def: CustomClientOptions,
+    ...options: CustomClientOptions[]
 ): AxiosRequestConfig {
-  const config: AxiosRequestConfig = {};
+    const config: AxiosRequestConfig = {};
 
-  config.baseURL = client.portalUrl;
+    config.baseURL = client.portalUrl;
 
-  const extraOptions = options.reduce((acc, val) => {
-    return {
-      ...acc,
-      ...val,
-    };
-  }, options);
+    const extraOptions = options.reduce((acc, val) => {
+        return {
+            ...acc,
+            ...val,
+        };
+    }, options);
 
-  const finalOptions = {
-    ...def,
-    ...client.customOptions,
-    ...extraOptions,
-  };
+    const finalOptions = {
+        ...def,
+        ...client.customOptions,
+        ...extraOptions,
+    } as CustomClientOptions;
 
-  if (finalOptions?.onDownloadProgress) {
-    config.onDownloadProgress = finalOptions?.onDownloadProgress;
-  }
+    if (finalOptions?.onDownloadProgress) {
+        config.onDownloadProgress = finalOptions?.onDownloadProgress;
+    }
 
-  if (finalOptions?.onUploadProgress) {
-    config.onUploadProgress = finalOptions?.onUploadProgress;
-  }
+    if (finalOptions?.onUploadProgress) {
+        config.onUploadProgress = finalOptions?.onUploadProgress;
+    }
 
-  if (finalOptions?.customCookie) {
-    config.headers = {
-      Cookie: finalOptions?.customCookie,
-    };
-  }
+    const headers = new AxiosHeaders(config.headers as AxiosHeaders)
 
-  if (finalOptions?.customUserAgent) {
-    config.headers = {
-      ...config.headers,
-      "User-Agent": finalOptions?.customUserAgent,
-    };
-  }
+    if (finalOptions?.customCookie) {
+        headers.set("Cookie", finalOptions.customCookie);
 
-  if (finalOptions?.ApiKey) {
-    config.headers = {
-      ...config.headers,
-      Authorization: `Bearer ${finalOptions?.ApiKey}`,
-    };
+    }
+    if (finalOptions?.customUserAgent) {
+        headers.set("User-Agent", finalOptions.customUserAgent);
+    }
 
-    config.withCredentials = true;
+    if (finalOptions?.apiKey) {
+        headers.set("Authorization", `Bearer ${finalOptions.apiKey}`);
+        config.withCredentials = true;
 
-    config.params = {
-      ...config.params,
-      auth_token: finalOptions?.ApiKey,
-    };
-  }
+        config.params = {
+            ...config.params,
+            auth_token: finalOptions?.apiKey,
+        };
+    }
 
-  return config;
+    config.headers = headers;
+
+    return config;
 }