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--- a/src/utils/base64.cpp
+++ b/src/utils/base64.cpp
@@ -40,7 +40,7 @@
 bool base64_decode(const string& in, string& out)
 {
     int io = 0, state = 0, ch = 0;
-    char *pos;
+    const char *pos;
     unsigned int ii = 0;
     out.erase();
     out.reserve(in.length());