diff --git a/reader.cpp b/reader.cpp
index cd8eb333b..392dee140 100644
--- a/reader.cpp
+++ b/reader.cpp
@@ -1648,7 +1648,7 @@ static bool handle_completions(const std::vector<completion_t> &comp)
         {
             // append just the end of the string
             prefix = wcstring(&ellipsis_char, 1);
-            prefix.append(data->command_line, prefix_start + len - PREFIX_MAX_LEN - 1, wcstring::npos);
+            prefix.append(data->command_line, prefix_start + len - PREFIX_MAX_LEN - 1, PREFIX_MAX_LEN);
         }
 
         {