A wrong variable in assignment?

I'am new to flutter, but what does this `String ` mean in line 192: 
` contentType: String ?? this.contentType,`?
This lead to an error after I called `merge()`.

dio/lib/src/options.dart

@@ -189,7 +189,7 @@ class Options extends _RequestConfig {
       extra: extra ?? Map.from(this.extra ?? {}),
       headers: headers ?? Map.from(this.headers ?? {}),
       responseType: responseType ?? this.responseType,
-      contentType: String ?? this.contentType,
+      contentType: contentType ?? this.contentType,
       validateStatus: validateStatus ?? this.validateStatus,
       receiveDataWhenStatusError:
           receiveDataWhenStatusError ?? this.receiveDataWhenStatusError,