diff --git a/README.md b/README.md
index 117b6a2..374d641 100644
--- a/README.md
+++ b/README.md
@@ -96,7 +96,7 @@ The module `pyfunnel.py` can also be run with the following command line interfa
 ```
 usage: pyfunnel.py [-h] --reference REFERENCE --test TEST [--output OUTPUT] [--atolx ATOLX] [--atoly ATOLY] [--ltolx LTOLX] [--ltoly LTOLY] [--rtolx RTOLX] [--rtoly RTOLY]
 
-Run funnel binary from terminal.
+Run funnel binary from terminal on two two-column CSV files.
 
 Output `errors.csv`, `lowerBound.csv`, `upperBound.csv`, `reference.csv`, `test.csv` into the output directory (`./results` by default).
 
diff --git a/pyfunnel/pyfunnel.py b/pyfunnel/pyfunnel.py
index 7e2eb11..7021b39 100755
--- a/pyfunnel/pyfunnel.py
+++ b/pyfunnel/pyfunnel.py
@@ -22,7 +22,7 @@
     parser = argparse.ArgumentParser(
         formatter_class=argparse.RawDescriptionHelpFormatter,
         description=(
-            'Run funnel binary from terminal.\n\n'
+            'Run funnel binary from terminal on two two-column CSV files.\n\n'
             'Output `errors.csv`, `lowerBound.csv`, `upperBound.csv`, `reference.csv`, `test.csv` '
             'into the output directory (`./results` by default).'),
         epilog='Full documentation at https://github.com/lbl-srg/funnel'
@@ -89,8 +89,8 @@
         data[s] = dict(x=[], y=[])
         with open(vars(args)[s]) as csvfile:
             spamreader = csv.reader(csvfile)
-            if (len(next(spamreader)) > 2):
-                raise RuntimeError("The {} CSV file cannot have more than two columns.".format(s))
+            if (len(next(spamreader)) != 2):
+                raise RuntimeError("The {} CSV file must have exactly two columns.".format(s))
             for row in spamreader:
                 try:
                     data[s]['x'].append(float(row[0]))