diff --git a/cores/esp32/libb64/cdecode.c b/cores/esp32/libb64/cdecode.c
index aa84ef60a8d3516613518e19bcf58b48cb23f7ae..47592b02ba8c14b22812c9564159265d5d6417df 100755
--- a/cores/esp32/libb64/cdecode.c
+++ b/cores/esp32/libb64/cdecode.c
@@ -12,7 +12,7 @@ static int base64_decode_value_signed(int8_t value_in){
   static const int8_t decoding[] = {62,-1,-1,-1,63,52,53,54,55,56,57,58,59,60,61,-1,-1,-1,-2,-1,-1,-1,0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,-1,-1,-1,-1,-1,-1,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51};
   static const int8_t decoding_size = sizeof(decoding);
   value_in -= 43;
-  if (value_in < 0 || value_in > decoding_size) return -1;
+  if (value_in < 0 || value_in >= decoding_size) return -1;
   return decoding[(int)value_in];
 }