diff --git a/permchain/pregel/validate.py b/permchain/pregel/validate.py
index 56aa31961..afa51834e 100644
--- a/permchain/pregel/validate.py
+++ b/permchain/pregel/validate.py
@@ -27,9 +27,14 @@ def validate_chains_channels(
             channels[chan] = LastValue(Any)
 
     if isinstance(input, str):
+        if input not in channels:
+            channels[input] = LastValue(Any)
         if input not in subscribed_channels:
             raise ValueError(f"Input channel {input} is not subscribed to by any chain")
     else:
+        for chan in input:
+            if chan not in channels:
+                channels[chan] = LastValue(Any)
         if all(chan not in subscribed_channels for chan in input):
             raise ValueError(
                 f"None of the input channels {input} are subscribed to by any chain"