diff --git a/src/atlas/util/PolygonLocator.h b/src/atlas/util/PolygonLocator.h
index a119ce303..b64749038 100644
--- a/src/atlas/util/PolygonLocator.h
+++ b/src/atlas/util/PolygonLocator.h
@@ -105,7 +105,7 @@ class PolygonLocator {
                     return ii;
                 }
             }
-            return -1;
+            return idx_t{-1};
         };
 
         if (partition < 0) {