diff --git a/vlib/builtin/string.v b/vlib/builtin/string.v
index 8b2981b181..63eb75df27 100644
--- a/vlib/builtin/string.v
+++ b/vlib/builtin/string.v
@@ -604,17 +604,17 @@ fn (s string) substr2(start int, _end int, end_max bool) string {
 // Example: assert 'ABCD'.substr(1,3) == 'BC'
 pub fn (s string) substr(start int, end int) string {
 	$if !no_bounds_checking ? {
-		/*
-		$if debug {
-			if start > end || start > s.len || end > s.len || start < 0 || end < 0 {
-				println('substr($start, $end) out of bounds (len=$s.len)')
-				println('s="$s"')
-				print_backtrace()
-				return ''
+		if start > end || start > s.len || end > s.len || start < 0 || end < 0 {
+			/*
+			$if debug {
+					println('substr($start, $end) out of bounds (len=$s.len)')
+					println('s="$s"')
+					print_backtrace()
+					return ''
 			}
+			*/
+			panic('substr($start, $end) out of bounds (len=$s.len)')
 		}
-		*/
-		panic('substr($start, $end) out of bounds (len=$s.len)')
 	}
 	len := end - start
 	if len == s.len {