diff --git a/procprof/make_example_images.py b/procprof/make_example_images.py
index 72904b1..8015c06 100755
--- a/procprof/make_example_images.py
+++ b/procprof/make_example_images.py
@@ -36,13 +36,19 @@ def mk_rand(n):
 def mk_real(fn, ch, n):
     ch = harmonize_channel(ch)
     with SFDataFiles(fn) as f:
-        imgs = f[ch][:n]
+        if ch is None:
+            names = list(f.names)
+            raise SystemExit(f"No camera or channel name specificed. Choose from: {names}")
+        c = f[ch]
+        print("Shape of raw images:", c.shape)
+        imgs = c[:n]
     nreal = len(imgs)
     if nreal != n:
         print(f"Warning: Got only {nreal} images from channel {ch} in {fn}")
     return imgs
 
 def harmonize_channel(ch):
+    if ch is None: return
     if not ch.endswith(FPIC):
         ch += FPIC
     return ch