diff --git a/mesonbuild/optinterpreter.py b/mesonbuild/optinterpreter.py
index 2d60d102b..5bddbc817 100644
--- a/mesonbuild/optinterpreter.py
+++ b/mesonbuild/optinterpreter.py
@@ -24,7 +24,7 @@ forbidden_option_names = set(coredata.builtin_options.keys())
 forbidden_prefixes = [lang + '_' for lang in compilers.all_languages] + ['b_', 'backend_']
 reserved_prefixes = ['cross_']
 
-def is_invalid_name(name):
+def is_invalid_name(name: str) -> bool:
     if name in forbidden_option_names:
         return True
     pref = name.split('_')[0] + '_'