diff --git a/org/qmckl_determinant.org b/org/qmckl_determinant.org
index 12cebf0..16cafed 100644
--- a/org/qmckl_determinant.org
+++ b/org/qmckl_determinant.org
@@ -307,7 +307,7 @@ int64_t qmckl_get_determinant_det_num_beta (const qmckl_context context) {
 
 int64_t* qmckl_get_determinant_mo_index_alpha (const qmckl_context context) {
   if (qmckl_context_check(context) == QMCKL_NULL_CONTEXT) {
-    return (int64_t) 0;
+    return NULL;
   }
 
   qmckl_context_struct* const ctx = (qmckl_context_struct* const) context;
@@ -325,7 +325,7 @@ int64_t* qmckl_get_determinant_mo_index_alpha (const qmckl_context context) {
 
 int64_t* qmckl_get_determinant_mo_index_beta (const qmckl_context context) {
   if (qmckl_context_check(context) == QMCKL_NULL_CONTEXT) {
-    return (int64_t) 0;
+    return NULL;
   }
 
   qmckl_context_struct* const ctx = (qmckl_context_struct* const) context;
@@ -334,7 +334,7 @@ int64_t* qmckl_get_determinant_mo_index_beta (const qmckl_context context) {
   int32_t mask = 1 << 5;
 
   if ( (ctx->det.uninitialized & mask) != 0) {
-    return (int64_t) 0;
+    return NULL;
   }
 
   assert (ctx->det.mo_index_beta != NULL);