diff --git a/dice/roller.py b/dice/roller.py
index a03c722..a65192c 100644
--- a/dice/roller.py
+++ b/dice/roller.py
@@ -93,12 +93,8 @@ class DiceRoller(object):
                     size = m[1]
                     if keep > dice or keep == 0:
                         keep = dice
-                    if size < 1:
-                        output = "# of sides for die is incorrect: %d" % size
-                        return output
-                    if dice < 1:
-                        output = "# of dice is incorrect: %d" % dice
-                        return output
+                    assert size >= 1, f"Die must have at least one side."
+                    assert dice >= 1, f"At least one die must be rolled."
                     res = self.roll_dice(keep, dice, size)
                     curr_str += "%d%s" % (res[0], res[1])
                     res = res[0]