diff --git a/.gitignore b/.gitignore
index ba9db1a..34abdb8 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,2 +1,3 @@
 *.pyc
 dist/
+*.egg-info/
diff --git a/rrl.py b/rrl.py
index 7a64629..ae70243 100644
--- a/rrl.py
+++ b/rrl.py
@@ -52,12 +52,15 @@ class RateLimiter:
         self.track_daily_usage = track_daily_usage
 
     def check_limit(self, zone: str, key: str, tier_name: str) -> bool:
+        try:
+            tier = self.tiers[tier_name]
+        except KeyError:
+            raise ValueError(f"unknown tier: {tier_name}")
         if self.use_redis_time:
             timestamp = self.redis.time()[0]
             now = datetime.datetime.fromtimestamp(timestamp)
         else:
             now = datetime.datetime.utcnow()
-        tier = self.tiers[tier_name]
 
         pipe = self.redis.pipeline()
         if tier.per_minute:
diff --git a/test_ratelimit.py b/test_ratelimit.py
index 8faf29a..84de977 100644
--- a/test_ratelimit.py
+++ b/test_ratelimit.py
@@ -58,6 +58,14 @@ def test_using_redis_time():
     assert count == 10
 
 
+def test_invalid_tier():
+    redis.flushall()
+    rl = RateLimiter(tiers=[simple_daily_tier], use_redis_time=True)
+
+    with pytest.raises(ValueError):
+        rl.check_limit("test-zone", "test-key", "non-existent-tier")
+
+
 def test_multiple_zones():
     redis.flushall()
     rl = RateLimiter(tiers=[simple_daily_tier], use_redis_time=True)