diff --git a/library/base64.c b/library/base64.c
index a58717d..9677dee 100644
--- a/library/base64.c
+++ b/library/base64.c
@@ -116,7 +116,7 @@
         *p++ = '=';
     }
 
-    *olen = p - dst;
+    *olen = (size_t) (p - dst);
     *p = 0;
 
     return 0;
@@ -225,7 +225,7 @@
         }
     }
 
-    *olen = p - dst;
+    *olen = (size_t) (p - dst);
 
     return 0;
 }