diff --git a/src/common/base64.h b/src/common/base64.h
index 31b02d3ca2a3..3f530e10a7e5 100644
--- a/src/common/base64.h
+++ b/src/common/base64.h
@@ -58,10 +58,10 @@ class StreamBufferReader {
   /*!
    * \return allows quick read using get char
    */
-  char GetChar() {
+  int GetChar() {
     while (true) {
       if (read_ptr_ < read_len_) {
-        return buffer_[read_ptr_++];
+        return static_cast<int>(buffer_[read_ptr_++]);
       } else {
         read_len_ = stream_->Read(&buffer_[0], buffer_.length());
         if (read_len_ == 0) return EOF;