diff --git a/arcparse/parser.py b/arcparse/parser.py
index 59ab7de..294f21a 100644
--- a/arcparse/parser.py
+++ b/arcparse/parser.py
@@ -16,8 +16,11 @@ def _extract_optional_type(typehint: type) -> type | None:
         return get_args(typehint)[0]
     elif origin in {Union, UnionType}:
         args = get_args(typehint)
-        if len(args) == 2 and args[1] == NoneType:
-            return args[0]
+        if len(args) == 2:
+            if args[0] == NoneType:
+                return args[1]
+            elif args[1] == NoneType:
+                return args[0]
     return None
 
 
@@ -205,12 +208,21 @@ def __collect_arguments(cls) -> tuple[dict[str, type], dict[str, _BaseArgument]]
             if isinstance(default, _Subparsers):
                 continue
 
-            if isinstance(default, _BaseArgument):
+            if get_origin(typehint) in {Union, UnionType}:
+                union_args = get_args(typehint)
+                if len(union_args) > 2 or NoneType not in union_args:
+                    raise Exception("Union can be used only for optional arguments (length of 2, 1 of them being None)")
+
+            if isinstance(default, _BaseValueArgument) and _extract_type_from_typehint(typehint) == bool:
+                raise Exception("Unable to make type=bool, everything would be True")
+            elif isinstance(default, _BaseArgument):
                 argument = default
             else:
                 typ = _extract_type_from_typehint(typehint)
 
                 if typ is bool:
+                    if _extract_optional_type(typehint):
+                        raise Exception("Unable to make type=bool, everything would be True")
                     argument = _Flag(default=default)
                 elif isinstance(typ, StrEnum):
                     argument = _Option(default=default, choices=list(typ), converter=typ)