diff --git a/sys/src/cmd/gs/src/zht2.c b/sys/src/cmd/gs/src/zht2.c
index c53653baf..c77fabd5d 100644
--- a/sys/src/cmd/gs/src/zht2.c
+++ b/sys/src/cmd/gs/src/zht2.c
@@ -81,20 +81,26 @@ zsethalftone5(i_ctx_t *i_ctx_p)
     gs_memory_t *mem;
     uint edepth = ref_stack_count(&e_stack);
     int npop = 2;
-    int dict_enum = dict_first(op);
+    int dict_enum;
     ref rvalue[2];
     int cname, colorant_number;
     byte * pname;
     uint name_size;
     int halftonetype, type = 0;
     gs_state *pgs = igs;
-    int space_index = r_space_index(op - 1);
+    int space_index;
+
+    if (ref_stack_count(&o_stack) < 2)
+        return_error(e_stackunderflow);
+    check_type(*op, t_dictionary);
+    check_type(*(op - 1), t_dictionary);
+
+    dict_enum = dict_first(op);
+    space_index = r_space_index(op - 1);
 
     mem = (gs_memory_t *) idmemory->spaces_indexed[space_index];
 
-    check_type(*op, t_dictionary);
     check_dict_read(*op);
-    check_type(op[-1], t_dictionary);
     check_dict_read(op[-1]);
  
     /*