diff --git a/b64.h b/b64.h
index e39d746..830f206 100644
--- a/b64.h
+++ b/b64.h
@@ -10,7 +10,7 @@
 
 typedef struct b64_buffer {
     char * ptr;
-    int bufc;
+    size_t bufc;
 } b64_buffer_t;
 
 /**