diff --git a/src/osgDB/ConvertBase64.cpp b/src/osgDB/ConvertBase64.cpp
index 50befdf66..3f5c1c7e8 100644
--- a/src/osgDB/ConvertBase64.cpp
+++ b/src/osgDB/ConvertBase64.cpp
@@ -31,7 +31,7 @@ namespace osgDB
         static const char 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 char 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];
     }