diff --git a/packages/core/src/lib/prepareParams.ts b/packages/core/src/lib/prepareParams.ts
index 40e7b1ae..bd7df2d2 100644
--- a/packages/core/src/lib/prepareParams.ts
+++ b/packages/core/src/lib/prepareParams.ts
@@ -356,7 +356,7 @@ export default async function prepareParams(
           if (metadata?.[paramName]) delete metadata[paramName];
           break;
         case 'header':
-          (params.header as NonNullable<typeof params.header>)[paramName.toLowerCase()] = value;
+          (params.header as NonNullable<typeof params.header>)[paramName] = value;
           if (metadataHeaderParam && metadata?.[metadataHeaderParam]) delete metadata[metadataHeaderParam];
           break;
         case 'cookie':
@@ -393,7 +393,7 @@ export default async function prepareParams(
             if (typeof metadata === 'object') {
               // this if-statement below is a typeguard
               if (typeof params.header === 'object') {
-                params.header[headerName] = metadata[headerParam] as string;
+                params.header[headerParam] = metadata[headerParam] as string;
               }
               // eslint-disable-next-line no-param-reassign
               delete metadata[headerParam];