From fca2947bc8f9d22db54abeb6cdf0465a699cd146 Mon Sep 17 00:00:00 2001
From: wpbonelli <wbonelli@ucar.edu>
Date: Mon, 8 Jun 2026 12:32:37 -0700
Subject: [PATCH 1/2] fix(mfdataplist): raise proper MFDataException on wrong
 col count

---
 autotest/test_mf6.py          | 24 ++++++++++++++++++++++++
 flopy/mf6/data/mfdataplist.py |  2 +-
 2 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/autotest/test_mf6.py b/autotest/test_mf6.py
index 4dd29ad5c..2a782fb61 100644
--- a/autotest/test_mf6.py
+++ b/autotest/test_mf6.py
@@ -4,6 +4,7 @@
 from shutil import copytree, which
 
 import numpy as np
+import pandas as pd
 import pytest
 from modflow_devtools.markers import requires_exe, requires_pkg
 from modflow_devtools.misc import set_dir
@@ -57,6 +58,7 @@
     PackageDimensions,
 )
 from flopy.mf6.data.mffileaccess import MFFileAccessArray
+from flopy.mf6.mfbase import MFDataException
 from flopy.mf6.data.mfstructure import MFDataItemStructure, MFDataStructure
 from flopy.mf6.mfsimbase import MFSimulationData
 from flopy.mf6.modflow import (
@@ -1450,6 +1452,28 @@ def test_get_set_data_record(function_tmpdir):
     sim.write_simulation()
 
 
+def test_set_data_dataframe_column_mismatch_error(function_tmpdir):
+    """
+    DataFrame with a wrong column count should raise MFDataException.
+    Previously the error message code referenced len(data[0]), which on a
+    DataFrame accesses the column named 0 and raises KeyError instead.
+    """
+    sim = MFSimulation(sim_ws=str(function_tmpdir), exe_name="mf6")
+    ModflowTdis(sim, nper=1, perioddata=[(1.0, 1, 1.0)])
+    ModflowIms(sim)
+    gwf = ModflowGwf(sim, modelname="gwf")
+    ModflowGwfdis(gwf, nlay=1, nrow=10, ncol=10)
+    ModflowGwfic(gwf, strt=0.0)
+    ModflowGwfnpf(gwf)
+    ModflowGwfwel(gwf, stress_period_data={0: [[(0, 0, 0), -1.0]]})
+    spd = gwf.get_package("WEL").stress_period_data
+
+    # 3 columns: matches neither _header_names (4) nor _data_item_names (2)
+    df = pd.DataFrame({"a": [0], "b": [3], "c": [-500.0]})
+    with pytest.raises(MFDataException):
+        spd.set_data({0: df})
+
+
 @requires_exe("mf6")
 def test_output(function_tmpdir, example_data_path):
     ex_name = "test001e_UZF_3lay"
diff --git a/flopy/mf6/data/mfdataplist.py b/flopy/mf6/data/mfdataplist.py
index e2377d490..c5adfb6e4 100644
--- a/flopy/mf6/data/mfdataplist.py
+++ b/flopy/mf6/data/mfdataplist.py
@@ -757,7 +757,7 @@ def set_data(self, data, autofill=False, check_data=True, append=False):
                 message = (
                     f"ERROR: Data list {self._data_name} supplied the "
                     f"wrong number of columns of data, expected "
-                    f"{len(self._data_item_names)} got {len(data[0])}.\n"
+                    f"{len(self._data_item_names)} got {len(data.columns)}.\n"
                     f"Data columns supplied: {data.columns}\n"
                     f"Data columns expected: {self._header_names}"
                 )

From 725128bc07dd33465ab119bd5d56fc30116cc275 Mon Sep 17 00:00:00 2001
From: wpbonelli <wbonelli@ucar.edu>
Date: Mon, 8 Jun 2026 12:38:28 -0700
Subject: [PATCH 2/2] ruff

---
 autotest/test_mf6.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/autotest/test_mf6.py b/autotest/test_mf6.py
index 2a782fb61..3b0912b11 100644
--- a/autotest/test_mf6.py
+++ b/autotest/test_mf6.py
@@ -58,8 +58,8 @@
     PackageDimensions,
 )
 from flopy.mf6.data.mffileaccess import MFFileAccessArray
-from flopy.mf6.mfbase import MFDataException
 from flopy.mf6.data.mfstructure import MFDataItemStructure, MFDataStructure
+from flopy.mf6.mfbase import MFDataException
 from flopy.mf6.mfsimbase import MFSimulationData
 from flopy.mf6.modflow import (
     mfgwf,