diff --git a/darwin/util.m b/darwin/util.m
index 6861147e..7d331fb4 100644
--- a/darwin/util.m
+++ b/darwin/util.m
@@ -21,5 +21,10 @@ void disableAutocorrect(NSTextView *tv)
 
 void complain(const char *fmt, ...)
 {
-	// TODO
+	va_list ap;
+
+	va_start(ap, fmt);
+	vfprintf(stderr, fmt, ap);
+	va_end(ap);
+	abort();
 }