diff --git a/src/compiler/nir/nir_lower_tex.c b/src/compiler/nir/nir_lower_tex.c
index 6c8c6aa1164..191076f8c27 100644
--- a/src/compiler/nir/nir_lower_tex.c
+++ b/src/compiler/nir/nir_lower_tex.c
@@ -934,6 +934,12 @@ nir_lower_tex_block(nir_block *block, nir_builder *b,
       nir_tex_instr *tex = nir_instr_as_tex(instr);
       bool lower_txp = !!(options->lower_txp & (1 << tex->sampler_dim));
 
+      /* we aren't really going to do the right thing if we haven't
+       * lowered tex/samp derefs yet:
+       */
+      assume(nir_tex_instr_src_index(tex, nir_tex_src_texture_deref) < 0);
+      assume(nir_tex_instr_src_index(tex, nir_tex_src_sampler_deref) < 0);
+
       /* mask of src coords to saturate (clamp): */
       unsigned sat_mask = 0;