diff --git a/pykern/fconf.py b/pykern/fconf.py
index 7c611d77..38aaa090 100644
--- a/pykern/fconf.py
+++ b/pykern/fconf.py
@@ -207,7 +207,7 @@ def __init__(self, files, base_vars=None):
                 pykern.pkinspect.append_exception_reason(e, f"fconf.Parser.file={f}")
                 raise
         if "yml" not in self.files:
-            raise ValueError("must supply at least on '.yml' file")
+            raise ValueError("must supply at least one '.yml' file")
         e = _Evaluator(
             global_fvars=PKDict() if base_vars is None else copy.deepcopy(base_vars),
             parser=self,