diff --git a/src/mesa/drivers/dri/i965/brw_fs.cpp b/src/mesa/drivers/dri/i965/brw_fs.cpp
index 2f23a9c7aac..fa95c818e98 100644
--- a/src/mesa/drivers/dri/i965/brw_fs.cpp
+++ b/src/mesa/drivers/dri/i965/brw_fs.cpp
@@ -1759,16 +1759,25 @@ fs_visitor::compact_virtual_grfs()
    }
 
    /* Patch all the references to delta_x/delta_y, since they're used in
-    * register allocation.
+    * register allocation.  If they're unused, switch them to BAD_FILE so
+    * we don't think some random VGRF is delta_x/delta_y.
     */
    for (unsigned i = 0; i < ARRAY_SIZE(delta_x); i++) {
-      if (delta_x[i].file == GRF && remap_table[delta_x[i].reg] != -1) {
-         delta_x[i].reg = remap_table[delta_x[i].reg];
+      if (delta_x[i].file == GRF) {
+         if (remap_table[delta_x[i].reg] != -1) {
+            delta_x[i].reg = remap_table[delta_x[i].reg];
+         } else {
+            delta_x[i].file = BAD_FILE;
+         }
       }
    }
    for (unsigned i = 0; i < ARRAY_SIZE(delta_y); i++) {
-      if (delta_y[i].file == GRF && remap_table[delta_y[i].reg] != -1) {
-         delta_y[i].reg = remap_table[delta_y[i].reg];
+      if (delta_y[i].file == GRF) {
+         if (remap_table[delta_y[i].reg] != -1) {
+            delta_y[i].reg = remap_table[delta_y[i].reg];
+         } else {
+            delta_y[i].file = BAD_FILE;
+         }
       }
    }
 }
diff --git a/src/mesa/drivers/dri/i965/brw_fs_reg_allocate.cpp b/src/mesa/drivers/dri/i965/brw_fs_reg_allocate.cpp
index a9bff65ea01..237cc497433 100644
--- a/src/mesa/drivers/dri/i965/brw_fs_reg_allocate.cpp
+++ b/src/mesa/drivers/dri/i965/brw_fs_reg_allocate.cpp
@@ -459,6 +459,7 @@ fs_visitor::assign_regs(bool allow_spilling)
        * that register and set it to the appropriate class.
        */
       if (screen->wm_reg_sets[rsi].aligned_pairs_class >= 0 &&
+          this->delta_x[BRW_WM_PERSPECTIVE_PIXEL_BARYCENTRIC].file == GRF &&
           this->delta_x[BRW_WM_PERSPECTIVE_PIXEL_BARYCENTRIC].reg == i) {
          c = screen->wm_reg_sets[rsi].aligned_pairs_class;
       }