diff --git a/src/zopfli/cache.c b/src/zopfli/cache.c
index 6b9444f0..e5934df4 100644
--- a/src/zopfli/cache.c
+++ b/src/zopfli/cache.c
@@ -33,8 +33,8 @@ void ZopfliInitCache(size_t blocksize, ZopfliLongestMatchCache* lmc) {
   lmc->sublen = (unsigned char*)malloc(ZOPFLI_CACHE_LENGTH * 3 * blocksize);
   if(lmc->sublen == NULL) {
     fprintf(stderr,
-        "Error: Out of memory. Tried allocating %lu bytes of memory.\n",
-        (unsigned long)ZOPFLI_CACHE_LENGTH * 3 * blocksize);
+        "Error: Out of memory. Tried allocating %zu bytes of memory.\n",
+        ZOPFLI_CACHE_LENGTH * 3 * blocksize);
     exit (EXIT_FAILURE);
   }