what if someone used a tuple of mappings?

Just be safe and check against ComponentMapping

also guard against len 0 list case

openfe/protocols/openmm_rfe/equil_rfe_methods.py

@@ -203,7 +203,7 @@ def _validate_alchemical_components(
     """
     # Check mapping
     # For now we only allow for a single mapping, this will likely change
-    if mapping is None or (not isinstance(mapping, ComponentMapping) and len(mapping) > 1):
+    if mapping is None or (not isinstance(mapping, ComponentMapping) and len(mapping) != 1):
         errmsg = "A single LigandAtomMapping is expected for this Protocol"
         raise ValueError(errmsg)