diff --git a/molter/command.py b/molter/command.py
index 6cffe67..360b04a 100644
--- a/molter/command.py
+++ b/molter/command.py
@@ -15,13 +15,24 @@
 class CommandParameter:
     name: str = attr.ib(default=None)
     default: typing.Optional[typing.Any] = attr.ib(default=None)
+    type: type = attr.ib(default=None)
     converters: list[
         typing.Callable[[dis_snek.MessageContext, str], typing.Any]
     ] = attr.ib(factory=list)
+    union: bool = attr.ib(default=False)
     variable: bool = attr.ib(default=False)
     consume_rest: bool = attr.ib(default=False)
 
 
+def _get_type_name(x: type):
+    try:
+        return x.__name__
+    except AttributeError:
+        if hasattr(x, "__origin__"):
+            return repr(x)
+        return x.__class__.__name__
+
+
 def _convert_to_bool(argument: str) -> bool:
     lowered = argument.lower()
     if lowered in ("yes", "y", "true", "t", "1", "enable", "on"):
@@ -71,9 +82,10 @@ def _get_params(func: typing.Callable):
             else dis_snek.const.MISSING
         )
 
-        anno = param.annotation
+        cmd_param.type = anno = param.annotation
 
         if typing.get_origin(anno) in {typing.Union, UnionType}:
+            cmd_param.union = True
             for arg in typing.get_args(anno):
                 if arg != NoneType:
                     converter = _get_converter(arg)
@@ -147,7 +159,10 @@ async def call_callback(
                             converted = await maybe_coroutine(converter, ctx, arg)
                             break
                         except Exception as e:
-                            if param.default == dis_snek.const.MISSING:
+                            if (
+                                not param.union
+                                and param.default == dis_snek.const.MISSING
+                            ):
                                 raise errors.BadArgument(str(e))
 
                     if converted == dis_snek.const.MISSING:
@@ -156,10 +171,13 @@ async def call_callback(
                             new_args.append(converted)
                             param_index += 1
                         else:
-                            # vague, ik
+                            union_types = typing.get_args(param.type)
+                            union_names = tuple(_get_type_name(t) for t in union_types)
+                            union_types_str = (
+                                ", ".join(union_names[:-1]) + f", or {union_names[-1]}"
+                            )
                             raise errors.BadArgument(
-                                f"Could not convert {arg} into a type specified for"
-                                f" {param.name}."
+                                f"Could not convert {arg} into {union_types_str}."
                             )
                     else:
                         new_args.append(converted)
diff --git a/molter/errors.py b/molter/errors.py
index c18dc47..eef18e6 100644
--- a/molter/errors.py
+++ b/molter/errors.py
@@ -1,5 +1,12 @@
+import typing
+
 import dis_snek
 
 
 class BadArgument(dis_snek.CommandException):
-    pass
+    def __init__(self, message: typing.Optional[str] = None, *args: typing.Any) -> None:
+        if message is not None:
+            message = dis_snek.utils.escape_mentions(message)
+            super().__init__(message, *args)
+        else:
+            super().__init__(*args)