diff --git a/src/engine.c b/src/engine.c
index dfad486cd53d68a64a18a58bf0ce71aa4fa5e0dd..f9b49c81129f693dcbcd4323a053ae59459a2d69 100644
--- a/src/engine.c
+++ b/src/engine.c
@@ -2341,10 +2341,10 @@ void engine_step(struct engine *e) {
 #ifdef SWIFT_GRAVITY_FORCE_CHECKS
   /* Check how many gparts are active this timestep. */
   size_t nr_gparts = e->s->nr_gparts;
-  long long gpart_active_count = 0;
+  size_t gpart_active_count = 0;
 
   /* Count active gparts */
-  for (long long i=0; i < nr_gparts; ++i) {
+  for (size_t i=0; i < nr_gparts; ++i) {
     struct gpart *gp = &e->s->gparts[i];
     if (gpart_is_active(gp, e)) gpart_active_count += 1;
   }