diff --git a/src/main/java/org/gcube/informationsystem/types/impl/properties/PropertyDefinitionImpl.java b/src/main/java/org/gcube/informationsystem/types/impl/properties/PropertyDefinitionImpl.java
index 15e6e5c..19e43af 100644
--- a/src/main/java/org/gcube/informationsystem/types/impl/properties/PropertyDefinitionImpl.java
+++ b/src/main/java/org/gcube/informationsystem/types/impl/properties/PropertyDefinitionImpl.java
@@ -262,9 +262,19 @@ public final class PropertyDefinitionImpl implements PropertyDefinition {
 			return ret;
 		}
 		
-		ret = regexp.compareTo(o.regexp);
-		if(ret != 0) {
-			return ret;
+		if(regexp==null && o.regexp!=null) {
+			return -1;
+		}
+		
+		if(o.regexp==null && regexp!=null) {
+			return -1;
+		}
+		
+		if(!(regexp==null && o.regexp==null)) {
+			ret = regexp.compareTo(o.regexp);
+			if(ret != 0) {
+				return ret;
+			}
 		}
 		
 		return propertyTypeName.toString().compareTo(o.propertyTypeName.toString());