diff --git a/src/RedirectablePrint.cpp b/src/RedirectablePrint.cpp
index 672c8334c..6ff7bbb18 100644
--- a/src/RedirectablePrint.cpp
+++ b/src/RedirectablePrint.cpp
@@ -345,7 +345,7 @@ void RedirectablePrint::log(const char *logLevel, const char *format, ...)
     return;
 }
 
-void RedirectablePrint::hexDump(const char *logLevel, unsigned char *buf, uint16_t len)
+void RedirectablePrint::hexDump(const char *logLevel, const unsigned char *buf, uint16_t len)
 {
     const char alphabet[17] = "0123456789abcdef";
     log(logLevel, "    +------------------------------------------------+ +----------------+");
diff --git a/src/RedirectablePrint.h b/src/RedirectablePrint.h
index 45b62b7af..c66226171 100644
--- a/src/RedirectablePrint.h
+++ b/src/RedirectablePrint.h
@@ -44,7 +44,7 @@ class RedirectablePrint : public Print
     /** like printf but va_list based */
     size_t vprintf(const char *logLevel, const char *format, va_list arg);
 
-    void hexDump(const char *logLevel, unsigned char *buf, uint16_t len);
+    void hexDump(const char *logLevel, const unsigned char *buf, uint16_t len);
 
     std::string mt_sprintf(const std::string fmt_str, ...);