diff --git a/src/libstore/store-api.cc b/src/libstore/store-api.cc
index 23631a3b8e..ced5fd0072 100644
--- a/src/libstore/store-api.cc
+++ b/src/libstore/store-api.cc
@@ -5,6 +5,12 @@
 #include <limits.h>
 
 
+/* Needed for some ancient environments. */
+#ifndef ULLONG_MAX
+#define ULLONG_MAX 18446744073709551615
+#endif
+
+
 namespace nix {