diff --git a/Sources/Fetch/Network/Resource.swift b/Sources/Fetch/Network/Resource.swift
index 92ca409..66681c2 100644
--- a/Sources/Fetch/Network/Resource.swift
+++ b/Sources/Fetch/Network/Resource.swift
@@ -158,8 +158,10 @@ open class Resource<T: Decodable>: CacheableResource {
         if let body = body {
             let (data, contentType) = try encode(body: body)
             urlRequest.httpBody = data
-            if let contentType = contentType {
-                 urlRequest.addValue(contentType.description, forHTTPHeaderField: "Content-Type")
+            
+            if headers["Content-Type"] == nil,
+               let contentType = contentType {
+                urlRequest.addValue(contentType.description, forHTTPHeaderField: "Content-Type")
             }
         }