diff --git a/src/parse_tree.cpp b/src/parse_tree.cpp
index 52bb3ea9b..3dc16ff26 100644
--- a/src/parse_tree.cpp
+++ b/src/parse_tree.cpp
@@ -134,14 +134,11 @@ wcstring parse_error_t::describe(const wcstring &src, bool is_interactive) const
 }
 
 void parse_error_offset_source_start(parse_error_list_t *errors, size_t amt) {
-    assert(errors != nullptr);
-    if (amt > 0) {
-        size_t i, max = errors->size();
-        for (i = 0; i < max; i++) {
-            parse_error_t *error = &errors->at(i);
+    if (amt > 0 && errors != nullptr) {
+        for (parse_error_t &error : *errors) {
             // Preserve the special meaning of -1 as 'unknown'.
-            if (error->source_start != SOURCE_LOCATION_UNKNOWN) {
-                error->source_start += amt;
+            if (error.source_start != SOURCE_LOCATION_UNKNOWN) {
+                error.source_start += amt;
             }
         }
     }