diff --git a/pyproject.toml b/pyproject.toml
index 90b11c06..b92cdb18 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -5,9 +5,7 @@ build-backend = "hatchling.build"
 [project]
 name = "formulaic"
 description = "An implementation of Wilkinson formulas."
-authors = [
-    {name = "Matthew Wardrop", email = "mpwardrop@gmail.com"},
-]
+authors = [{ name = "Matthew Wardrop", email = "mpwardrop@gmail.com" }]
 license = "MIT"
 readme = "README.md"
 classifiers = [
@@ -105,22 +103,15 @@ dependencies = [
 ]
 
 [tool.hatch.envs.lint]
-dependencies = [
-    "mypy==1.11.1",
-    "mypy-extensions==1.0.0",
-    "ruff==0.5.6",
-]
+dependencies = ["mypy==1.11.1", "mypy-extensions==1.0.0", "ruff==0.5.6"]
 
 [tool.hatch.envs.lint.scripts]
-check = [
-    "ruff check",
-    "ruff format --check",
-    "mypy formulaic",
-]
-format = [
-    "ruff check --fix",
-    "ruff format",
-]
+check = ["ruff check", "ruff format --check", "mypy formulaic"]
+format = ["ruff check --fix", "ruff format"]
+
+# Coverage configuration
+[tool.coverage.run]
+omit = ["formulaic/_version.py"]
 
 # Linting configuration
 
@@ -141,11 +132,11 @@ exclude = [
 
 [tool.ruff.lint]
 select = [
-    "F",  # flake8 rules
-    "E",  # pycodestyle
-    "W",  # pycodestyle
-    "S",  # bandit
-    "I001",  # import sorting
+    "F",    # flake8 rules
+    "E",    # pycodestyle
+    "W",    # pycodestyle
+    "S",    # bandit
+    "I001", # import sorting
     # "D", # documentation
 ]
 
@@ -156,7 +147,7 @@ ignore = [
     "E712",
     "E722",
     "E731",
-    "S307",  # Use of ast.literal_eval
+    "S307", # Use of ast.literal_eval
 ]
 
 [tool.ruff.lint.per-file-ignores]
diff --git a/tests/parser/types/test_operator_resolver.py b/tests/parser/types/test_operator_resolver.py
index aca53366..3a7b6370 100644
--- a/tests/parser/types/test_operator_resolver.py
+++ b/tests/parser/types/test_operator_resolver.py
@@ -1,3 +1,5 @@
+import re
+
 import pytest
 
 from formulaic.errors import FormulaParsingError, FormulaSyntaxError
@@ -28,3 +30,6 @@ def resolver(self):
     def test_resolve(self, resolver):
         assert list(resolver.resolve(Token("+")))[0][1][0] is OPERATOR_PLUS
         assert list(resolver.resolve(Token("-")))[0][1][0] is OPERATOR_UNARY_MINUS
+
+        with pytest.raises(FormulaSyntaxError, match=re.escape("Unknown operator '&'")):
+            list(resolver.resolve(Token("&")))
diff --git a/tests/transforms/test_scale.py b/tests/transforms/test_scale.py
index 098029fe..53ce2abf 100644
--- a/tests/transforms/test_scale.py
+++ b/tests/transforms/test_scale.py
@@ -1,3 +1,5 @@
+import re
+
 import numpy
 import pytest
 import scipy.sparse as spsparse
@@ -41,6 +43,13 @@ def test_scale():
     )
     assert state == {"center": 1, "scale": 2, "ddof": 1}
 
+    # Make sure we raise for sparse matrices with more than one column
+    with pytest.raises(
+        ValueError,
+        match=re.escape("Cannot scale a sparse matrix with more than one column."),
+    ):
+        scale(data=spsparse.csc_matrix([[1, 2, 3], [4, 5, 6]]), center=[1, 2, 3])
+
 
 def test_center():
     state = {}
diff --git a/tests/utils/test_calculus.py b/tests/utils/test_calculus.py
index b7122c33..62b9f933 100644
--- a/tests/utils/test_calculus.py
+++ b/tests/utils/test_calculus.py
@@ -1,7 +1,9 @@
+import re
+
 import pytest
 
 from formulaic.parser.types import Factor, Term
-from formulaic.utils.calculus import differentiate_term
+from formulaic.utils.calculus import differentiate_term, _differentiate_factors
 
 
 def test_differentiate_term():
@@ -21,3 +23,12 @@ def test_differentiate_term_with_sympy():
         str(differentiate_term(t, ["log(b)"], use_sympy=True)) == "0"
     )  # 'log(b)' is not in the free symbol list.
     assert str(differentiate_term(t, ["b"], use_sympy=True)) == "a:(1/b)"
+
+
+def test__differentiate_factors():
+    t = Term([Factor("a"), Factor("log(a)")])
+
+    assert _differentiate_factors(t.factors, "a", use_sympy=True) == {"(log(a) + 1)"}
+
+    with pytest.raises(RuntimeError, match=re.escape("Cannot differentiate non-trivial factors without `sympy`.")):
+        _differentiate_factors(t.factors, "a", use_sympy=False)