diff --git a/src/libstore/store-api.cc b/src/libstore/store-api.cc
index bd330a6da9..e80be1e68c 100644
--- a/src/libstore/store-api.cc
+++ b/src/libstore/store-api.cc
@@ -7,7 +7,7 @@
 
 /* Needed for some ancient environments. */
 #ifndef ULLONG_MAX
-#define ULLONG_MAX 18446744073709551615
+#define ULLONG_MAX 18446744073709551615ULL
 #endif