Index: trunk/psModules/src/detrend/pmDark.c
===================================================================
--- trunk/psModules/src/detrend/pmDark.c	(revision 17049)
+++ trunk/psModules/src/detrend/pmDark.c	(revision 17054)
@@ -337,5 +337,5 @@
     }
     float norm = NAN;                   // Normalisation value
-    if (normConcept) {
+    if (normConcept && strlen(normConcept) > 0) {
         if (!ordinateLookup(&norm, normConcept, false, NAN, NAN, readout)) {
             psError(PS_ERR_UNKNOWN, true, "Unable to find value for %s", normConcept);