diff --git a/src/main/java/leon/codegen/runtime/LeonCodeGenRuntimeHenkinMonitor.java b/src/main/java/leon/codegen/runtime/LeonCodeGenRuntimeHenkinMonitor.java
index 62e7a7b3bbc3053b4515159f1814f213edbc3a58..0be4ad91212be930ec5f8730ed30ebcf9a9f4e0a 100644
--- a/src/main/java/leon/codegen/runtime/LeonCodeGenRuntimeHenkinMonitor.java
+++ b/src/main/java/leon/codegen/runtime/LeonCodeGenRuntimeHenkinMonitor.java
@@ -26,7 +26,8 @@ public class LeonCodeGenRuntimeHenkinMonitor extends LeonCodeGenRuntimeMonitor {
       }
     }
 
-    domain.addAll(domains.get(type));
+    List<Tuple> tpeDomain = domains.get(type);
+    if (tpeDomain != null) domain.addAll(tpeDomain);
 
     return domain;
   }