diff --git a/src/libCStreamedXML/defaults.c b/src/libCStreamedXML/defaults.c
index b4d147e..a02e2f1 100644
--- a/src/libCStreamedXML/defaults.c
+++ b/src/libCStreamedXML/defaults.c
@@ -15,7 +15,7 @@ static void default_notWellFormed(const struct csxml* ctx, const char* reason)
 static void default_outOfMemory(const struct csxml* ctx, size_t amount)
 {
     (void)ctx;
-    fprintf(stderr, "Streamed XML parser: out of memory allocating %lu bytes\n", amount);
+    fprintf(stderr, "Streamed XML parser: out of memory allocating %lu bytes\n", (unsigned long)amount);
 }