Index: trunk/psModules/src/imcombine/pmReadoutCombine.c
===================================================================
--- trunk/psModules/src/imcombine/pmReadoutCombine.c	(revision 18861)
+++ trunk/psModules/src/imcombine/pmReadoutCombine.c	(revision 19086)
@@ -63,14 +63,4 @@
                 "ROBUST_MEDIAN, FITTED_MEAN or CLIPPED_MEAN.\n");
         return false;
-    }
-
-    // weights exist if weights desired?
-    for (int i = 0; i < inputs->n; i++) {
-        pmReadout *readout = inputs->data[i]; // Readout of interest
-        if (params->weights && !readout->weight) {
-            psError(PS_ERR_UNEXPECTED_NULL, true,
-                    "Rejection based on weights requested, but no weights supplied for image %d.\n", i);
-            return false;
-        }
     }
 
@@ -179,4 +169,18 @@
     PS_ASSERT_FLOAT_WITHIN_RANGE(params->fracLow, 0.0, 1.0, false);
     PS_ASSERT_FLOAT_WITHIN_RANGE(params->fracHigh, 0.0, 1.0, false);
+
+    // does required/desired data exist?
+    for (int i = 0; i < inputs->n; i++) {
+        pmReadout *readout = inputs->data[i]; // Readout of interest
+        if (!readout->image) {
+            psError(PS_ERR_UNEXPECTED_NULL, true, "Image data is missing for image %d.\n", i);
+            return false;
+        }
+        if (params->weights && !readout->weight) {
+            psError(PS_ERR_UNEXPECTED_NULL, true,
+                    "Rejection based on weights requested, but no weights supplied for image %d.\n", i);
+            return false;
+        }
+    }
 
     pmHDU *hdu = pmHDUFromReadout(output); // Output HDU