diff --git a/bstrd/bscache.py b/bstrd/bscache.py
index 8620366..c8dbfac 100644
--- a/bstrd/bscache.py
+++ b/bstrd/bscache.py
@@ -4,6 +4,12 @@ from .bsvar import BSVar
 from .prodthread import ProdThread
 
 
+FIXED_CHANNELS = {
+    "pid",
+    "toc"
+}
+
+
 class BSCache:
 
     def __init__(self, timeout=1000, maxsize=100):
@@ -51,7 +57,7 @@ class BSCache:
 
         new_chans = {}
         for name, kwargs in names.items():
-            if name not in ["pid", "toc"] and name not in self.channels:
+            if name not in FIXED_CHANNELS and name not in self.channels:
                 if not is_available(name):
                     raise ValueError(f"channel {name} is not available")
                 cfg = make_channel_config(name, *kwargs)
@@ -72,13 +78,13 @@ class BSCache:
             next(self)
 
         while not chans.keys() <= self.data.keys():
-            missing = self.data.keys() - chans.keys() - {"pid", "toc"}
+            missing = self.data.keys() - chans.keys() - FIXED_CHANNELS
             print("dropping data that is missing new channel(s)", sorted(missing))
             next(self)
 
 
     def get_var(self, name, modulo=None, offset=None):
-        if name not in ["pid", "toc"] and name not in self.channels:
+        if name not in FIXED_CHANNELS and name not in self.channels:
             if not is_available(name):
                 raise ValueError(f"channel {name} is not available")
             cfg = make_channel_config(name, modulo, offset)