--- a/media/gmp-clearkey/0.1/ClearKeyUtils.cpp
+++ b/media/gmp-clearkey/0.1/ClearKeyUtils.cpp
@@ -92,17 +92,17 @@ EncodeBase64Web(vector<uint8_t> aBinary,
   // Pad binary data in case there's rubbish past the last byte.
   aBinary.push_back(0);
 
   // Number of bytes not consumed in the previous character
   uint32_t shift = 0;
 
   auto out = aEncoded.begin();
   auto data = aBinary.begin();
-  for (int i = 0; i < aEncoded.length(); i++) {
+  for (string::size_type i = 0; i < aEncoded.length(); i++) {
     if (shift) {
       out[i] = (*data << (6 - shift)) & sMask;
       data++;
     } else {
       out[i] = 0;
     }
 
     out[i] += (*data >> (shift + 2)) & sMask;