diff --git a/library/pem.c b/library/pem.c
index 8c8f39d..aed4788 100644
--- a/library/pem.c
+++ b/library/pem.c
@@ -491,7 +491,7 @@
     size_t len = 0, use_len, add_len = 0;
 
     mbedtls_base64_encode(NULL, 0, &use_len, der_data, der_len);
-    add_len = strlen(header) + strlen(footer) + (use_len / 64) + 1;
+    add_len = strlen(header) + strlen(footer) + (((use_len > 2) ? (use_len - 2) : 0) / 64) + 1;
 
     if (use_len + add_len > buf_len) {
         *olen = use_len + add_len;