diff --git a/rpc/js/src/dial.ts b/rpc/js/src/dial.ts
index 73182baf..f68208ab 100644
--- a/rpc/js/src/dial.ts
+++ b/rpc/js/src/dial.ts
@@ -228,19 +228,20 @@ class authenticatedTransport implements Transport {
     method: MethodInfo<I, O>,
     signal: AbortSignal | undefined,
     timeoutMs: number | undefined,
-    header: Headers,
+    header: HeadersInit | undefined,
     message: PartialMessage<I>,
     contextValues?: ContextValues
   ): Promise<UnaryResponse<I, O>> {
-    this.extraHeaders.forEach((key: string, value: string) => {
-      header.set(key, value);
+    let newHeaders = cloneHeaders(header);
+    this.extraHeaders.forEach((value: string, key: string) => {
+      newHeaders.set(key, value);
     });
     return this.transport.unary(
       service,
       method,
       signal,
       timeoutMs,
-      header,
+      newHeaders,
       message,
       contextValues
     );
@@ -254,25 +255,50 @@ class authenticatedTransport implements Transport {
     method: MethodInfo<I, O>,
     signal: AbortSignal | undefined,
     timeoutMs: number | undefined,
-    header: Headers,
+    header: HeadersInit | undefined,
     input: AsyncIterable<PartialMessage<I>>,
     contextValues?: ContextValues
   ): Promise<StreamResponse<I, O>> {
-    this.extraHeaders.forEach((key: string, value: string) => {
-      header.set(key, value);
+    let newHeaders = cloneHeaders(header);
+    this.extraHeaders.forEach((value: string, key: string) => {
+      newHeaders.set(key, value);
     });
     return this.transport.stream(
       service,
       method,
       signal,
       timeoutMs,
-      header,
+      newHeaders,
       input,
       contextValues
     );
   }
 }
 
+function cloneHeaders(headers: HeadersInit | undefined): Headers {
+  let cloned = new Headers();
+  if (headers) {
+    if (headers !== undefined) {
+      if (Array.isArray(headers)) {
+        for (const [key, value] of headers) {
+          cloned.append(key, value);
+        }
+      } else if ("forEach" in headers) {
+        if (typeof headers.forEach == "function") {
+          headers.forEach((value, key) => {
+            cloned.append(key, value);
+          });
+        }
+      } else {
+        for (const [key, value] of Object.entries<string>(headers)) {
+          cloned.append(key, value);
+        }
+      }
+    }
+  }
+  return cloned;
+}
+
 export interface WebRTCConnection {
   transport: Transport;
   peerConnection: RTCPeerConnection;