diff --git a/sys/base64/base64.c b/sys/base64/base64.c
index 849ba8ca21da6e1c670fd65d9e9cb40d3c52d551..0bafec5c81d51a402ba99a55d4d3d99563bcd299 100644
--- a/sys/base64/base64.c
+++ b/sys/base64/base64.c
@@ -83,9 +83,9 @@ int base64_encode(unsigned char *data_in, size_t data_in_size, \
     unsigned char nNum = 0;
     int nLst = 0;
     int njump = 0;
-    unsigned char tmpval;
 
     for (int i = 0; i < (int)(data_in_size); ++i) {
+        unsigned char tmpval;
         njump++;
         tmpval = *(data_in + i);