diff --git a/packages/connect-node/src/node-universal-header.ts b/packages/connect-node/src/node-universal-header.ts
index 5a130f052..a0d1e5191 100644
--- a/packages/connect-node/src/node-universal-header.ts
+++ b/packages/connect-node/src/node-universal-header.ts
@@ -71,22 +71,30 @@ export function webHeaderToNodeHeaders(
     return undefined;
   }
   const o = Object.create(null) as http.OutgoingHttpHeaders;
+  const append = (key: string, value: string): void => {
+    key = key.toLowerCase();
+    const existing = o[key];
+    if (typeof existing == "string") {
+      o[key] = [existing, value];
+    } else if (Array.isArray(existing)) {
+      existing.push(value);
+    } else {
+      o[key] = value;
+    }
+  };
   if (Array.isArray(headersInit)) {
     for (const [key, value] of headersInit) {
-      const k = key.toLowerCase();
-      o[k] = value;
+      append(key, value);
     }
   } else if ("forEach" in headersInit) {
     if (typeof headersInit.forEach == "function") {
       headersInit.forEach((value, key) => {
-        const k = key.toLowerCase();
-        o[k] = value;
+        append(key, value);
       });
     }
   } else {
     for (const [key, value] of Object.entries<string>(headersInit)) {
-      const k = key.toLowerCase();
-      o[k] = value;
+      append(key, value);
     }
   }
   return o;