diff --git a/config/windows.config b/config/windows.config
index 923fca1a9..1d50506e9 100644
--- a/config/windows.config
+++ b/config/windows.config
@@ -121,7 +121,7 @@ if variants.uwp:
 
 # https://github.com/actions/runner-images/issues/10004#issuecomment-2156109231
 msvc_env_for_build_system['CFLAGS'] += ['-D_DISABLE_CONSTEXPR_MUTEX_CONSTRUCTOR']
-msvc_env_for_build_system['CXXFLAGS'] += ['D_DISABLE_CONSTEXPR_MUTEX_CONSTRUCTOR']
+msvc_env_for_build_system['CXXFLAGS'] += ['-D_DISABLE_CONSTEXPR_MUTEX_CONSTRUCTOR']
 
 # When not cross-compiling, this is set using C{,PLUS}_INCLUDE_PATH in cerbero/config.py
 if cross: