diff --git a/jetty-server/src/main/java/org/eclipse/jetty/server/Response.java b/jetty-server/src/main/java/org/eclipse/jetty/server/Response.java
index 5a3ed15e570c..e023a1f1c78a 100644
--- a/jetty-server/src/main/java/org/eclipse/jetty/server/Response.java
+++ b/jetty-server/src/main/java/org/eclipse/jetty/server/Response.java
@@ -1017,11 +1017,9 @@ public void setContentType(String contentType)
             _contentType = contentType;
             _mimeType = MimeTypes.CACHE.get(contentType);
 
-            String charset;
-            if (_mimeType != null && _mimeType.getCharset() != null && !_mimeType.isCharsetAssumed())
+            String charset = MimeTypes.getCharsetFromContentType(contentType);
+            if (charset == null && _mimeType != null && _mimeType.isCharsetAssumed())
                 charset = _mimeType.getCharsetString();
-            else
-                charset = MimeTypes.getCharsetFromContentType(contentType);
 
             if (charset == null)
             {
@@ -1056,10 +1054,10 @@ public void setContentType(String contentType)
             else if (isWriting() && !charset.equalsIgnoreCase(_characterEncoding))
             {
                 // too late to change the character encoding;
-                _mimeType = null;
                 _contentType = MimeTypes.getContentTypeWithoutCharset(_contentType);
-                if (_characterEncoding != null)
+                if (_characterEncoding != null  && (_mimeType == null || !_mimeType.isCharsetAssumed()))
                     _contentType = _contentType + ";charset=" + _characterEncoding;
+                _mimeType = MimeTypes.CACHE.get(_contentType);
             }
             else
             {
diff --git a/jetty-servlet/src/test/java/org/eclipse/jetty/servlet/ResponseHeadersTest.java b/jetty-servlet/src/test/java/org/eclipse/jetty/servlet/ResponseHeadersTest.java
index 4e7275961c18..81145690ef2a 100644
--- a/jetty-servlet/src/test/java/org/eclipse/jetty/servlet/ResponseHeadersTest.java
+++ b/jetty-servlet/src/test/java/org/eclipse/jetty/servlet/ResponseHeadersTest.java
@@ -258,7 +258,7 @@ public void testCharsetChangeToJsonMimeTypeSetCharsetToNull() throws Exception
     {
         HttpTester.Request request = new HttpTester.Request();
         request.setMethod("GET");
-        request.setURI("/charset/json-reset-null/");
+        request.setURI("/charset/json-change-null/");
         request.setVersion(HttpVersion.HTTP_1_1);
         request.setHeader("Connection", "close");
         request.setHeader("Host", "test");