diff --git a/src/mesa/state_tracker/st_pbo_compute.c b/src/mesa/state_tracker/st_pbo_compute.c
index a52233f911d..ade01432f36 100644
--- a/src/mesa/state_tracker/st_pbo_compute.c
+++ b/src/mesa/state_tracker/st_pbo_compute.c
@@ -620,10 +620,19 @@ create_conversion_shader(struct st_context *st, enum pipe_texture_target target,
    nir_ssa_def *iid = nir_load_local_invocation_id(&b);
    nir_ssa_def *tile = nir_imul(&b, wid, bsize);
    nir_ssa_def *global_id = nir_iadd(&b, tile, iid);
-   nir_ssa_def *start = nir_iadd(&b, global_id, sd.offset);
+   nir_ssa_def *start = nir_iadd(&b, nir_trim_vector(&b, global_id, 2), sd.offset);
 
-   nir_ssa_def *coord = nir_channels(&b, start, (1<<coord_components)-1);
-   nir_ssa_def *max = nir_iadd(&b, sd.offset, sd.range);
+   nir_ssa_def *coord;
+   if (coord_components < 3)
+      coord = start;
+   else {
+      /* pad offset vec with global_id to get correct z offset */
+      assert(coord_components == 3);
+      coord = nir_vec3(&b, nir_channel(&b, start, 0),
+                           nir_channel(&b, start, 1),
+                           nir_channel(&b, global_id, 2));
+   }
+   nir_ssa_def *max = nir_iadd(&b, nir_pad_vector_imm_int(&b, sd.offset, 0, 3), sd.range);
    nir_push_if(&b, nir_ball(&b, nir_ilt(&b, coord, nir_trim_vector(&b, max, coord_components))));
    nir_tex_instr *txf = nir_tex_instr_create(b.shader, 3);
    txf->is_array = glsl_sampler_type_is_array(sampler->type);