diff --git a/models/jellyfish.cc b/models/jellyfish.cc
index 79be9ef..f3851aa 100644
--- a/models/jellyfish.cc
+++ b/models/jellyfish.cc
@@ -375,7 +375,7 @@ void lake_t::draw_density(TCL_args args)
         if (njellies[i]>maxj) maxj=njellies[i];
       }
 
-  vector<unsigned char> pp(3*width*height,255);
+  vector<unsigned char> pp(size_t(3)*width*height,255);
   Tk_PhotoImageBlock block={pp.data(),int(width),int(height),int(width)*3,3,{0,1,2}};
   float invlogmaxj=1/log(maxj+1);
   for (unsigned x=0; x<width; x++)