Index: trunk/psModules/src/astrom/pmConceptsWrite.c
===================================================================
--- trunk/psModules/src/astrom/pmConceptsWrite.c	(revision 6888)
+++ trunk/psModules/src/astrom/pmConceptsWrite.c	(revision 6896)
@@ -129,4 +129,7 @@
                              )
 {
+    if (!hdu->header) {
+        return false;
+    }
     switch (item->type) {
     case PS_DATA_STRING:
@@ -166,4 +169,8 @@
     if (item->type == PS_DATA_LIST) {
         psList *values = item->data.V;  // List of outputs
+        if (values->n == 0) {
+            // Nothing to write
+            return false;
+        }
         psList *keys = psStringSplit(keywords, " ,;"); // List of keywords
         if (keys->n != values->n) {