diff --git a/src/java/davmail/exchange/VProperty.java b/src/java/davmail/exchange/VProperty.java
index 9c44c4dc..bea6b56f 100644
--- a/src/java/davmail/exchange/VProperty.java
+++ b/src/java/davmail/exchange/VProperty.java
@@ -279,9 +279,11 @@ public class VProperty {
      * @param paramValue param value
      */
     public void addParam(String paramName, String paramValue) {
-        List<String> paramValues = new ArrayList<String>();
-        paramValues.add(paramValue);
-        addParam(paramName, paramValues);
+        if (paramValue != null) {
+            List<String> paramValues = new ArrayList<String>();
+            paramValues.add(paramValue);
+            addParam(paramName, paramValues);
+        }
     }
 
     protected void addParam(String paramName, List<String> paramValues) {