diff --git a/src/tests/sunsynk/test_helpers.py b/src/tests/sunsynk/test_helpers.py
index 7eac583f..6f4591d8 100644
--- a/src/tests/sunsynk/test_helpers.py
+++ b/src/tests/sunsynk/test_helpers.py
@@ -1,5 +1,7 @@
 """Test helpers."""
 
+import struct
+
 import pytest
 
 from sunsynk.helpers import (
@@ -119,6 +121,12 @@ def test_pack_unpack() -> None:
     assert unpack_value(pack_value(val, bits=16, signed=True), signed=True) == val
     assert unpack_value(pack_value(val, bits=32, signed=True), signed=True) == val
 
+    # Test error cases
+    with pytest.raises(struct.error):
+        pack_value(-1, bits=16, signed=False)
+    with pytest.raises(ValueError):
+        pack_value(1, bits=8)  # Invalid bit length
+
 
 def test_patch_bitmask() -> None:
     """Test patch_bitmask function."""
diff --git a/src/tests/sunsynk/test_rwsensors.py b/src/tests/sunsynk/test_rwsensors.py
index de303f10..720added 100644
--- a/src/tests/sunsynk/test_rwsensors.py
+++ b/src/tests/sunsynk/test_rwsensors.py
@@ -94,6 +94,7 @@ def test_number_rw(state: InverterState) -> None:
 
     # writing negative values (when allowed by min)
     s.min = -10
+    s.factor = -1  # indicate signed values
     assert s.value_to_reg(-1, state.get) == (65535,)
 
     s = NumberRWSensor(1, "s2", factor=0.01)