diff --git a/README.md b/README.md
index aa489fc..374d641 100644
--- a/README.md
+++ b/README.md
@@ -96,24 +96,23 @@ 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).
 
 optional arguments:
-  -h, --help            show this help message and exit
-  --output OUTPUT       Path of directory to store output data
-  --atolx ATOLX         Absolute tolerance along x axis
-  --atoly ATOLY         Absolute tolerance along y axis
-  --ltolx LTOLX         Relative tolerance along x axis (relatively to the local value)
-  --ltoly LTOLY         Relative tolerance along y axis (relatively to the local value)
-  --rtolx RTOLX         Relative tolerance along x axis (relatively to the range)
-  --rtoly RTOLY         Relative tolerance along y axis (relatively to the range)
+  -h, --help              show this help message and exit
+  --output OUTPUT         Path of directory to store output data
+  --atolx ATOLX           Absolute tolerance along x axis
+  --atoly ATOLY           Absolute tolerance along y axis
+  --ltolx LTOLX           Relative tolerance along x axis (relatively to the local value)
+  --ltoly LTOLY           Relative tolerance along y axis (relatively to the local value)
+  --rtolx RTOLX           Relative tolerance along x axis (relatively to the range)
+  --rtoly RTOLY           Relative tolerance along y axis (relatively to the range)
 
 required named arguments:
-  --reference REFERENCE
-                        Path of CSV file with reference data
-  --test TEST           Path of CSV file with test data
+  --reference REFERENCE   Path of two-column CSV file with reference data
+  --test TEST             Path of two-column CSV file with test data
 
 Full documentation at https://github.com/lbl-srg/funnel
 ```
diff --git a/pyfunnel/pyfunnel.py b/pyfunnel/pyfunnel.py
index 057bb3d..7a5e83c 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'
@@ -31,12 +31,12 @@
 
     required_named.add_argument(
         "--reference",
-        help="Path of CSV file with reference data",
+        help="Path of two-column CSV file with reference data",
         required=True
     )
     required_named.add_argument(
         "--test",
-        help="Path of CSV file with test data",
+        help="Path of two-column CSV file with test data",
         required=True
     )
     parser.add_argument(
@@ -89,7 +89,13 @@
         data[s] = dict(x=[], y=[])
         with open(vars(args)[s]) as csvfile:
             spamreader = csv.reader(csvfile)
-            for row in spamreader:
+            for i, row in enumerate(spamreader):
+                if (l := len(row)) != 2:
+                    raise IOError(
+                        "The {} CSV file must have exactly two columns. Row {} contains {} elements.".format(
+                            s, i, l
+                        )
+                    )
                 try:
                     data[s]['x'].append(float(row[0]))
                     data[s]['y'].append(float(row[1]))