diff --git a/src/mesa/state_tracker/st_program.c b/src/mesa/state_tracker/st_program.c
index 6802bb3b06d..1852228b29b 100644
--- a/src/mesa/state_tracker/st_program.c
+++ b/src/mesa/state_tracker/st_program.c
@@ -167,6 +167,7 @@ st_translate_vertex_program(struct st_context *st,
 
          switch (attr) {
          case VERT_RESULT_HPOS:
+            assert(slot == 0);
             vs.output_semantic_name[slot] = TGSI_SEMANTIC_POSITION;
             vs.output_semantic_index[slot] = 0;
             break;
@@ -218,6 +219,8 @@ st_translate_vertex_program(struct st_context *st,
       }
    }
 
+   assert(vs.output_semantic_name[0] == TGSI_SEMANTIC_POSITION);
+
 
    if (outputMapping) {
       /* find max output slot referenced to compute vs.num_outputs */