diff --git a/prism/src/hybrid/PrismHybrid.cc b/prism/src/hybrid/PrismHybrid.cc
index fe0cc2a9..65fdc944 100644
--- a/prism/src/hybrid/PrismHybrid.cc
+++ b/prism/src/hybrid/PrismHybrid.cc
@@ -101,7 +101,9 @@ JNIEXPORT void JNICALL Java_hybrid_PrismHybrid_PH_1SetMainLog(JNIEnv *env, jclas
 	// if main log has been set previously, we need to delete existing global refs first
 	if (main_log_obj != NULL) {
 		env->DeleteGlobalRef(main_log_cls);
+		main_log_cls = NULL;
 		env->DeleteGlobalRef(main_log_obj);
+		main_log_obj = NULL;
 	}
 	
 	// make a global reference to the log object
@@ -119,7 +121,9 @@ JNIEXPORT void JNICALL Java_hybrid_PrismHybrid_PH_1SetTechLog(JNIEnv *env, jclas
 	// if tech log has been set previously, we need to delete existing global refs first
 	if (tech_log_obj != NULL) {
 		env->DeleteGlobalRef(tech_log_cls);
+		tech_log_cls = NULL;
 		env->DeleteGlobalRef(tech_log_obj);
+		tech_log_obj = NULL;
 	}
 	
 	// make a global reference to the log object
diff --git a/prism/src/mtbdd/PrismMTBDD.cc b/prism/src/mtbdd/PrismMTBDD.cc
index 200e5818..6786abe4 100644
--- a/prism/src/mtbdd/PrismMTBDD.cc
+++ b/prism/src/mtbdd/PrismMTBDD.cc
@@ -92,7 +92,9 @@ JNIEXPORT void JNICALL Java_mtbdd_PrismMTBDD_PM_1SetMainLog(JNIEnv *env, jclass
 	// if main log has been set previously, we need to delete existing global refs first
 	if (main_log_obj != NULL) {
 		env->DeleteGlobalRef(main_log_cls);
+		main_log_cls = NULL;
 		env->DeleteGlobalRef(main_log_obj);
+		main_log_obj = NULL;
 	}
 	
 	// make a global reference to the log object
@@ -110,7 +112,9 @@ JNIEXPORT void JNICALL Java_mtbdd_PrismMTBDD_PM_1SetTechLog(JNIEnv *env, jclass
 	// if tech log has been set previously, we need to delete existing global refs first
 	if (tech_log_obj != NULL) {
 		env->DeleteGlobalRef(tech_log_cls);
+		tech_log_cls = NULL;
 		env->DeleteGlobalRef(tech_log_obj);
+		tech_log_obj = NULL;
 	}
 	
 	// make a global reference to the log object
diff --git a/prism/src/sparse/PrismSparse.cc b/prism/src/sparse/PrismSparse.cc
index 361e8d46..6ee7ccb5 100644
--- a/prism/src/sparse/PrismSparse.cc
+++ b/prism/src/sparse/PrismSparse.cc
@@ -94,7 +94,9 @@ JNIEXPORT void JNICALL Java_sparse_PrismSparse_PS_1SetMainLog(JNIEnv *env, jclas
 	// if main log has been set previously, we need to delete existing global refs first
 	if (main_log_obj != NULL) {
 		env->DeleteGlobalRef(main_log_cls);
+		main_log_cls = NULL;
 		env->DeleteGlobalRef(main_log_obj);
+		main_log_obj = NULL;
 	}
 	
 	// make a global reference to the log object
@@ -112,7 +114,9 @@ JNIEXPORT void JNICALL Java_sparse_PrismSparse_PS_1SetTechLog(JNIEnv *env, jclas
 	// if tech log has been set previously, we need to delete existing global refs first
 	if (tech_log_obj != NULL) {
 		env->DeleteGlobalRef(tech_log_cls);
+		tech_log_cls = NULL;
 		env->DeleteGlobalRef(tech_log_obj);
+		tech_log_obj = NULL;
 	}
 	
 	// make a global reference to the log object