diff --git a/src/plugins/output_genericio.cc b/src/plugins/output_genericio.cc
index d12847d..fe58dd4 100644
--- a/src/plugins/output_genericio.cc
+++ b/src/plugins/output_genericio.cc
@@ -16,7 +16,7 @@ protected:
     std::vector<float> vx, vy, vz;
     std::vector<float> mass, hh, uu;
     std::vector<float> mu, phi, rho;
-    std::vector<int16_t> mask;
+    std::vector<uint16_t> mask;
 
 public:
     //! constructor