diff --git a/src/tools/OpenMP.h b/src/tools/OpenMP.h
index 7c7f7dcc28..7715ac47ff 100644
--- a/src/tools/OpenMP.h
+++ b/src/tools/OpenMP.h
@@ -54,7 +54,7 @@ class OpenMP {
 
 template<typename T>
 unsigned OpenMP::getGoodNumThreads(const T*x,unsigned n) {
-  unsigned long p=(unsigned long) x;
+  unsigned long long p=(unsigned long long) x;
   (void) p; // this is not to have warnings. notice that the pointer location is not used actually.
 // a factor two is necessary since there is no guarantee that x is aligned
 // to cache line boundary