diff --git a/src/space.c b/src/space.c
index 2e6df8d2fadf0674f9332afa2b86d1b37fdd8232..f488cb3d258981d6db121127c30e040b5a9a300a 100644
--- a/src/space.c
+++ b/src/space.c
@@ -488,7 +488,7 @@ void parts_sort(struct part *parts, struct xpart *xparts, int *ind, int N,
     volatile int ready;
   };
   struct qstack *qstack;
-  int qstack_size = 2 * (max - min) + 10;
+  unsigned int qstack_size = 2 * (max - min) + 10;
   volatile unsigned int first, last, waiting;
 
   int pivot;