diff --git a/paddle/strings/stringpiece.cc b/paddle/strings/stringpiece.cc
index 4f788c6ecd56166aa0520db139a6cb1a29fae7cd..415b3558d5dfffde26275bcb16ea3922424ca9f3 100644
--- a/paddle/strings/stringpiece.cc
+++ b/paddle/strings/stringpiece.cc
@@ -16,11 +16,11 @@
 
 #include "paddle/strings/stringpiece.h"
 
-// #include <stddef.h>
 #include <string.h>
 
 #include <algorithm>
 #include <iosfwd>
+#include <stdexcept>
 
 namespace paddle {
 
@@ -39,6 +39,12 @@ StringPiece::StringPiece(const char* s) : data_(s) {
 StringPiece::StringPiece(const std::string& s)
     : data_(s.data()), size_(s.size()) {}
 
+char StringPiece::operator[](size_t n) const {
+  if (n >= len())
+    throw std::invalid_argument("index out of StringPiece length");
+  return data_[n];
+}
+
 int Compare(StringPiece a, StringPiece b) {
   const size_t min_len = (a.len() < b.len()) ? a.len() : b.len();
   int r = memcmp(a.data(), b.data(), min_len);
diff --git a/paddle/strings/stringpiece.h b/paddle/strings/stringpiece.h
index 413b65d38412126deef10436770cdd888de6c60c..89aa084a2920c6f372b771e0e407b64728ea69e4 100644
--- a/paddle/strings/stringpiece.h
+++ b/paddle/strings/stringpiece.h
@@ -16,9 +16,6 @@
 
 #pragma once
 
-#include <assert.h>
-
-#include <stdexcept>
 #include <string>
 
 namespace paddle {
@@ -46,10 +43,7 @@ public:
   const char* data() const { return data_; }
   size_t len() const { return size_; }
 
-  char operator[](size_t n) const {
-    assert(n < len());
-    return data_[n];
-  }
+  char operator[](size_t n) const;
 
   // StringPiece doesn't own the string, so both iterator and const
   // iterator are const char* indeed.