diff --git a/prism/src/mtbdd/PM_ExportLabels.cc b/prism/src/mtbdd/PM_ExportLabels.cc
index 46d80a12..30f47c6a 100644
--- a/prism/src/mtbdd/PM_ExportLabels.cc
+++ b/prism/src/mtbdd/PM_ExportLabels.cc
@@ -127,7 +127,7 @@ jstring fn		// filename
 	
 	// close file, etc.
 	if (export_file) fclose(export_file);
-	env->ReleaseStringUTFChars(na, export_name);
+	if (na) env->ReleaseStringUTFChars(na, export_name);
 	
 	return 0;
 }
diff --git a/prism/src/mtbdd/PM_ExportMatrix.cc b/prism/src/mtbdd/PM_ExportMatrix.cc
index 5ac202e8..eb5b22b6 100644
--- a/prism/src/mtbdd/PM_ExportMatrix.cc
+++ b/prism/src/mtbdd/PM_ExportMatrix.cc
@@ -86,7 +86,7 @@ jstring fn		// filename
 	
 	// close file, etc.
 	if (export_file) fclose(export_file);
-	env->ReleaseStringUTFChars(na, export_name);
+	if (na) env->ReleaseStringUTFChars(na, export_name);
 	
 	return 0;
 }
diff --git a/prism/src/mtbdd/PM_ExportVector.cc b/prism/src/mtbdd/PM_ExportVector.cc
index 0f145497..3b1ed800 100644
--- a/prism/src/mtbdd/PM_ExportVector.cc
+++ b/prism/src/mtbdd/PM_ExportVector.cc
@@ -76,7 +76,7 @@ jstring fn		// filename
 	
 	// close file, etc.
 	if (export_file) fclose(export_file);
-	env->ReleaseStringUTFChars(na, export_name);
+	if (na) env->ReleaseStringUTFChars(na, export_name);
 	
 	return 0;
 }
diff --git a/prism/src/sparse/PS_ExportMatrix.cc b/prism/src/sparse/PS_ExportMatrix.cc
index e2a80eb6..07e3ee14 100644
--- a/prism/src/sparse/PS_ExportMatrix.cc
+++ b/prism/src/sparse/PS_ExportMatrix.cc
@@ -162,7 +162,7 @@ jstring fn		// filename
 	
 	// close file, etc.
 	if (export_file) fclose(export_file);
-	env->ReleaseStringUTFChars(na, export_name);
+	if (na) env->ReleaseStringUTFChars(na, export_name);
 	
 	// catch exceptions: return (undocumented) error code for memout
 	} catch (std::bad_alloc e) {
diff --git a/prism/src/sparse/PS_ExportSubMDP.cc b/prism/src/sparse/PS_ExportSubMDP.cc
index 007438e7..0c37d840 100644
--- a/prism/src/sparse/PS_ExportSubMDP.cc
+++ b/prism/src/sparse/PS_ExportSubMDP.cc
@@ -120,7 +120,7 @@ jstring fn		// filename
 	
 	// close file, etc.
 	if (export_file) fclose(export_file);
-	env->ReleaseStringUTFChars(na, export_name);
+	if (na) env->ReleaseStringUTFChars(na, export_name);
 	
 	// catch exceptions: return (undocumented) error code for memout
 	} catch (std::bad_alloc e) {