diff --git a/toolkit/scheduler.py b/toolkit/scheduler.py
index 2404ae69..ab87f68b 100644
--- a/toolkit/scheduler.py
+++ b/toolkit/scheduler.py
@@ -24,6 +24,11 @@ def get_lr_scheduler(
         # StepLR decays purely on step_size/gamma and has no notion of run
         # length, so drop the total_iters the trainer injects.
         kwargs.pop('total_iters', None)
+        if 'step_size' not in kwargs:
+            raise ValueError(
+                "lr_scheduler 'step' requires lr_scheduler_params.step_size "
+                "(number of steps between each lr decay)"
+            )
 
         return torch.optim.lr_scheduler.StepLR(
             optimizer, **kwargs