diff --git a/src/pg_rad/configs/defaults.py b/src/pg_rad/configs/defaults.py
index 97fae3d..2d45681 100644
--- a/src/pg_rad/configs/defaults.py
+++ b/src/pg_rad/configs/defaults.py
@@ -15,4 +15,4 @@ DEFAULT_MAX_TURN_ANGLE = 90.
 
 DEFAULT_FRICTION_COEFF = 0.7      # dry asphalt
 DEFAULT_GRAVITATIONAL_ACC = 9.81  # m/s^2
-DEFAULT_ALPHA = 100.
\ No newline at end of file
+DEFAULT_ALPHA = 100.
diff --git a/src/pg_rad/inputparser/parser.py b/src/pg_rad/inputparser/parser.py
index db1cc90..a566764 100644
--- a/src/pg_rad/inputparser/parser.py
+++ b/src/pg_rad/inputparser/parser.py
@@ -109,7 +109,10 @@ class ConfigParser:
             raise ValueError(
                 "options.air_density must be a positive float in kg/m^3."
             )
-        if not isinstance(seed, int) or seed < 0:
+        if (
+            seed is not None or
+            (isinstance(seed, int) and seed <= 0)
+        ):
             raise ValueError("Seed must be a positive integer value.")
 
         return SimulationOptionsSpec(