diff --git a/process/models/physics/bootstrap_current.py b/process/models/physics/bootstrap_current.py
index 0282f2b23..9524cd99a 100644
--- a/process/models/physics/bootstrap_current.py
+++ b/process/models/physics/bootstrap_current.py
@@ -1470,6 +1470,10 @@ def bootstrap_fraction_sauter(self, plasma_profile: float) -> float:
 
         # Calculate electron and ion temperature profiles
         tempe = plasma_profile.teprofile.profile_y
+        # Check for any negative temperature in profile: always fatal eventually,
+        # report explicitly at source
+        if (tempe < 0).any():
+            raise ValueError("Negative temperature in plasma profile")
         tempi = (
             physics_variables.temp_plasma_ion_vol_avg_kev
             / physics_variables.temp_plasma_electron_vol_avg_kev
diff --git a/process/models/physics/physics.py b/process/models/physics/physics.py
index 1c3b0c628..90a026a3a 100644
--- a/process/models/physics/physics.py
+++ b/process/models/physics/physics.py
@@ -1161,6 +1161,12 @@ def plasma_composition():
             - znimp
         )
 
+        # Negative znfuel can occur during solution and is always fatal,
+        # frequently resulting in a confusing bootstrap current error later.
+        # Catch early and explicitly instead
+        if znfuel < 0.0:
+            raise ValueError(f"znfuel is negative: {znfuel}")
+
         # ======================================================================
 
         # Fuel ion density, nd_plasma_fuel_ions_vol_avg