diff --git a/lib/serviceClient.ts b/lib/serviceClient.ts
index 83e5e4a..6b039ed 100644
--- a/lib/serviceClient.ts
+++ b/lib/serviceClient.ts
@@ -221,6 +221,10 @@ export class ServiceClient {
       }
       httpRequest.url = requestUrl.toString();
 
+      if (operationSpec.contentType) {
+        httpRequest.headers.set("Content-Type", operationSpec.contentType);
+      }
+
       if (operationSpec.headerParameters) {
         for (const headerParameter of operationSpec.headerParameters) {
           let headerValue: any = getOperationArgumentValueFromParameter(operationArguments, headerParameter, operationSpec.serializer);
@@ -238,10 +242,6 @@ export class ServiceClient {
         }
       }
 
-      if (operationSpec.contentType) {
-        httpRequest.headers.set("Content-Type", operationSpec.contentType);
-      }
-
       if (operationArguments.customHeaders) {
         for (const customHeaderName in operationArguments.customHeaders) {
           httpRequest.headers.set(customHeaderName, operationArguments.customHeaders[customHeaderName]);