diff --git a/src/evalfunc.c b/src/evalfunc.c
index d78589c..f310520 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -494,6 +494,7 @@
     if (type->tt_type == VAR_FUNC
 	    && !(type->tt_member->tt_type == VAR_BOOL
 		|| type->tt_member->tt_type == VAR_NUMBER
+		|| type->tt_member->tt_type == VAR_UNKNOWN
 		|| type->tt_member->tt_type == VAR_ANY))
     {
 	arg_type_mismatch(&t_func_bool, type, context->arg_idx + 1);