diff --git a/snakebids/cli.py b/snakebids/cli.py
index e6e4d676..ac4ce348 100644
--- a/snakebids/cli.py
+++ b/snakebids/cli.py
@@ -48,7 +48,7 @@ def __call__(
                 key = pair
                 value = None
             if ":" in key:
-                key, spec = cast("tuple[str, str]", pair.split(":", 1))
+                key, spec = cast("tuple[str, str]", key.split(":", 1))
                 spec = spec.lower()
                 if spec == "optional":
                     value = OptionalFilter