diff --git a/src/vim9expr.c b/src/vim9expr.c
index 31fa824..38a65d4 100644
--- a/src/vim9expr.c
+++ b/src/vim9expr.c
@@ -1950,7 +1950,7 @@
 			if (p[2] == 'n' && p[3] == 'o' && p[4] == 't')
 			    *len = 5;
 			i = p[*len];
-			if (!isalnum(i) && i != '_')
+			if (!SAFE_isalnum(i) && i != '_')
 			{
 			    type = *len == 2 ? EXPR_IS : EXPR_ISNOT;
 			    *type_is = TRUE;