diff --git a/src/main/java/org/gcube/data/analysis/dminvocation/DataMinerInvocationManager.java b/src/main/java/org/gcube/data/analysis/dminvocation/DataMinerInvocationManager.java
index 51a20bb..0dda27e 100644
--- a/src/main/java/org/gcube/data/analysis/dminvocation/DataMinerInvocationManager.java
+++ b/src/main/java/org/gcube/data/analysis/dminvocation/DataMinerInvocationManager.java
@@ -37,6 +37,9 @@ public class DataMinerInvocationManager {
 	    Marshaller jaxbMarshaller = jaxbContext.createMarshaller();
 	    jaxbMarshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, Boolean.TRUE);
 
+	    if(mediaType==null)
+	    	mediaType = MediaType.ApplicationXML;
+
 	    switch (mediaType) {
 		case ApplicationJSON:
 			jaxbMarshaller.setProperty(MarshallerProperties.JSON_INCLUDE_ROOT, Boolean.TRUE);
@@ -65,6 +68,8 @@ public class DataMinerInvocationManager {
 	    JAXBContext jaxbContext = JAXBContext.newInstance(DataMinerInvocation.class);
 	    Unmarshaller jaxbUnmarshaller = jaxbContext.createUnmarshaller();
 
+	    if(mediaType==null)
+	    	mediaType = MediaType.ApplicationXML;
 
 	    switch (mediaType) {
 		case ApplicationJSON: