diff --git a/src/Microsoft.AspNet.Mvc.Core/Formatters/StringOutputFormatter.cs b/src/Microsoft.AspNet.Mvc.Core/Formatters/StringOutputFormatter.cs
index 6a8bb81bee..494a807e99 100644
--- a/src/Microsoft.AspNet.Mvc.Core/Formatters/StringOutputFormatter.cs
+++ b/src/Microsoft.AspNet.Mvc.Core/Formatters/StringOutputFormatter.cs
@@ -34,8 +34,12 @@ public override bool CanWriteResult(OutputFormatterCanWriteContext context)
             // always return it as a text/plain format.
             if (context.ObjectType == typeof(string) || context.Object is string)
             {
-                context.ContentType = new StringSegment(SupportedMediaTypes[0]);
-                return true;
+                if (context.ContentType == null)
+                {
+                    context.ContentType = new StringSegment(SupportedMediaTypes[0]);
+                }
+            
+	    	return true;
             }
 
             return false;