Index: trunk/psModules/src/camera/pmFPAfileFitsIO.c
===================================================================
--- trunk/psModules/src/camera/pmFPAfileFitsIO.c	(revision 7589)
+++ trunk/psModules/src/camera/pmFPAfileFitsIO.c	(revision 7618)
@@ -101,5 +101,11 @@
         if (hdu != phu) {
             // we assume that the PHU is just a header
-            psMetadata *outhead = psMetadataCopy (NULL, phu->header);
+            // header may not be defined for constructed images; make a dummy one
+            psMetadata *outhead = NULL;
+            if (phu->header) {
+                outhead = psMetadataCopy (NULL, phu->header);
+            } else {
+                outhead = psMetadataAlloc ();
+            }
             psMetadataAdd (outhead, PS_LIST_TAIL, "EXTEND", PS_DATA_BOOL | PS_META_REPLACE, "this file has extensions", true);
             psFitsWriteBlank (file->fits, outhead);