diff --git a/prism/src/hybrid/PrismHybrid.cc b/prism/src/hybrid/PrismHybrid.cc
index 65fdc944..4de33541 100644
--- a/prism/src/hybrid/PrismHybrid.cc
+++ b/prism/src/hybrid/PrismHybrid.cc
@@ -318,9 +318,13 @@ JNIEXPORT void JNICALL Java_hybrid_PrismHybrid_PH_1FreeGlobalRefs(JNIEnv *env, j
 {
 	// delete all global references
 	env->DeleteGlobalRef(main_log_cls);
+	main_log_cls = NULL;
 	env->DeleteGlobalRef(tech_log_cls);
+	tech_log_cls = NULL;
 	env->DeleteGlobalRef(main_log_obj);
+	main_log_obj = NULL;
 	env->DeleteGlobalRef(tech_log_obj);
+	tech_log_obj = NULL;
 }
 
 //------------------------------------------------------------------------------
diff --git a/prism/src/mtbdd/PrismMTBDD.cc b/prism/src/mtbdd/PrismMTBDD.cc
index 6786abe4..2b339c42 100644
--- a/prism/src/mtbdd/PrismMTBDD.cc
+++ b/prism/src/mtbdd/PrismMTBDD.cc
@@ -281,9 +281,13 @@ JNIEXPORT void JNICALL Java_mtbdd_PrismMTBDD_PM_1FreeGlobalRefs(JNIEnv *env, jcl
 {
 	// delete all global references
 	env->DeleteGlobalRef(main_log_cls);
+	main_log_cls = NULL;
 	env->DeleteGlobalRef(tech_log_cls);
+	tech_log_cls = NULL;
 	env->DeleteGlobalRef(main_log_obj);
+	main_log_obj = NULL;
 	env->DeleteGlobalRef(tech_log_obj);
+	tech_log_obj = NULL;
 }
 
 //------------------------------------------------------------------------------
diff --git a/prism/src/sparse/PrismSparse.cc b/prism/src/sparse/PrismSparse.cc
index 6ee7ccb5..b88b5b24 100644
--- a/prism/src/sparse/PrismSparse.cc
+++ b/prism/src/sparse/PrismSparse.cc
@@ -315,9 +315,13 @@ JNIEXPORT void JNICALL Java_sparse_PrismSparse_PS_1FreeGlobalRefs(JNIEnv *env, j
 {
 	// delete all global references
 	env->DeleteGlobalRef(main_log_cls);
+	main_log_cls = NULL;
 	env->DeleteGlobalRef(tech_log_cls);
+	tech_log_cls = NULL;
 	env->DeleteGlobalRef(main_log_obj);
+	main_log_obj = NULL;
 	env->DeleteGlobalRef(tech_log_obj);
+	tech_log_obj = NULL;
 }
 
 //------------------------------------------------------------------------------