diff --git a/src/crypto_utils.c b/src/crypto_utils.c
index 9be06db..9fd9af4 100644
--- a/src/crypto_utils.c
+++ b/src/crypto_utils.c
@@ -313,6 +313,9 @@ int base64url_encode(unsigned char *dst, size_t dlen, size_t *olen, const unsign
             dst[i] = '_';
         }
     }
+    if (*olen == 0) {
+        return 0;
+    }
     uint8_t *p = dst + *olen - 1;
     while (*p == '=') {
         *p-- = '\0';
@@ -363,4 +366,4 @@ int b64url_decoded_len(size_t n, size_t *out_len) {
 
     *out_len = out;
     return 0;
-}
\ No newline at end of file
+}