diff --git a/cores/esp8266/MD5Builder.cpp b/cores/esp8266/MD5Builder.cpp
index 6539d1ae6..a382093a4 100644
--- a/cores/esp8266/MD5Builder.cpp
+++ b/cores/esp8266/MD5Builder.cpp
@@ -23,7 +23,7 @@ void MD5Builder::addHexString(const char * data){
   free(tmp);
 }
 
-bool MD5Builder::addStream(Stream & stream, const int total_len) {
+bool MD5Builder::addStream(Stream & stream, const size_t total_len) {
     const int buf_size = 512;
     int bytesleft = total_len;
     uint8_t * buf = (uint8_t*) malloc(buf_size);
diff --git a/cores/esp8266/MD5Builder.h b/cores/esp8266/MD5Builder.h
index 274aa0a55..ec1ac8311 100644
--- a/cores/esp8266/MD5Builder.h
+++ b/cores/esp8266/MD5Builder.h
@@ -37,7 +37,7 @@ class MD5Builder {
     void addHexString(const char * data);
     void addHexString(char * data){ addHexString((const char*)data); }
     void addHexString(String data){ addHexString(data.c_str()); }
-    bool addStream(Stream & stream, int len);
+    bool addStream(Stream & stream, const size_t total_len);
     void calculate(void);
     void getBytes(uint8_t * output);
     void getChars(char * output);