diff --git a/json/src/main/groovy/grails/plugin/json/view/test/TestRequestConfigurer.groovy b/json/src/main/groovy/grails/plugin/json/view/test/TestRequestConfigurer.groovy
index b56f3b144..754f6f525 100644
--- a/json/src/main/groovy/grails/plugin/json/view/test/TestRequestConfigurer.groovy
+++ b/json/src/main/groovy/grails/plugin/json/view/test/TestRequestConfigurer.groovy
@@ -26,7 +26,7 @@ class TestRequestConfigurer implements Request {
     String contentType = 'application/json'
     String characterEncoding = 'UTF-8'
     Map<String, List<String>> headers = new LinkedHashMap<String, List<String>>().withDefault { String name ->
-        return []
+        return [] as List<String>
     }
     Map<String, Object> attributes = new LinkedHashMap<String, Object>()