diff --git a/webcc/base64.cc b/webcc/base64.cc
index 17cb827..5f7b8c4 100644
--- a/webcc/base64.cc
+++ b/webcc/base64.cc
@@ -54,7 +54,7 @@ static const char kAlphabet[] =
     "0123456789+/";
 
 // kInverse['z'] -> 51 which is the index of 'z' in kAlphabet.
-static const char kInverse[] = {
+static const signed char kInverse[] = {
   -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,  //   0-15
   -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,  //  16-31
   -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 62, -1, -1, -1, 63,  //  32-47