diff --git a/.flake8 b/.flake8
index 5ff4b7c..5c3d2ba 100644
--- a/.flake8
+++ b/.flake8
@@ -4,7 +4,11 @@ count = True
 max-line-length = 88
 extend-ignore = E203
 exclude =
+    .venv,
+    venv,
     .vscode,
     .conda,
+    .venv,
+    tests.venv/,
     .hatch,
     __pycache__
\ No newline at end of file
diff --git a/bdms/mutators.py b/bdms/mutators.py
index 162ff7a..99fb0e3 100644
--- a/bdms/mutators.py
+++ b/bdms/mutators.py
@@ -200,6 +200,8 @@ def __init__(
                 f"Transition matrix {transition_matrix} is not a valid stochastic"
                 " matrix."
             )
+        if (not len(transition_matrix) == 1) and np.diag(transition_matrix).any():
+            raise ValueError("transition_matrix diagonal must be zero.")
         super().__init__(attr=attr)
         self.state_space = state_space
         self.state_space_idxs: dict[Any, int] = {
diff --git a/bdms/poisson.py b/bdms/poisson.py
index 0b4f794..22d6934 100644
--- a/bdms/poisson.py
+++ b/bdms/poisson.py
@@ -196,6 +196,8 @@ class ConstantProcess(HomogeneousProcess):
     def __init__(self, value: float = 1.0):
         super().__init__()
         self.value = value
+        if self.value < 0:
+            raise ValueError("The constant rate must be a non-negative float")
 
     def λ_homogeneous(
         self, x: Hashable | Sequence[Hashable] | NDArray[Any]
@@ -216,6 +218,14 @@ def __init__(
         self, rates: Mapping[Hashable, float] | Sequence[float], attr: str = "state"
     ):
         super().__init__(attr=attr)
+        if isinstance(rates, Mapping):
+            for rate in rates.values():
+                if rate < 0.0:
+                    raise ValueError("The rate for each state must be >= 0")
+        else:
+            for rate in rates:
+                if rate < 0.0:
+                    raise ValueError("The rate for each state must be >= 0")
         self.rates = rates
 
     def λ_homogeneous(
diff --git a/pyproject.toml b/pyproject.toml
index 7365981..a692a4f 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -51,6 +51,8 @@ packages = ["bdms"]
 [tool.hatch.version]
 path = "bdms/__init__.py"
 
+
+
 # this puts hatch envs in a .hatch directory, so vscode can find them
 
 
@@ -102,3 +104,6 @@ format = [
   "black .",
   "docformatter --black --in-place **/*.py",
 ]
+
+[tool.flake8]
+exclude = ".venv,venv"
\ No newline at end of file