diff --git a/elab_expr.cc b/elab_expr.cc
index 46e98f8db..1f5ac0f33 100644
--- a/elab_expr.cc
+++ b/elab_expr.cc
@@ -3981,6 +3981,7 @@ NetExpr* PEIdent::elaborate_expr_net_part_(Design*des, NetScope*scope,
 	    ivl_assert(*this, lrc);
 	    lrc = net->sig()->sb_to_slice(prefix_indices, msv, moff, mwid);
 	    ivl_assert(*this, lrc);
+	    ivl_assert(*this, lwid == mwid);
 
 	    if (moff > loff) {
 		  sb_lsb = loff;
@@ -3989,6 +3990,7 @@ NetExpr* PEIdent::elaborate_expr_net_part_(Design*des, NetScope*scope,
 		  sb_lsb = moff;
 		  sb_msb = loff + lwid - 1;
 	    }
+	    wid = sb_msb - sb_lsb + 1;
       } else {
 	      // This case, the prefix indices are enough to index
 	      // down to a single bit/slice.
diff --git a/nettypes.cc b/nettypes.cc
index 6b5797146..301f28603 100644
--- a/nettypes.cc
+++ b/nettypes.cc
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2012 Stephen Williams (steve@icarus.com)
+ * Copyright (c) 2012-2013 Stephen Williams (steve@icarus.com)
  *
  *    This source code is free software; you can redistribute it
  *    and/or modify it in source code form under the terms of the GNU
@@ -98,7 +98,7 @@ bool prefix_to_slice(const std::vector<netrange_t>&dims,
       if (pcur->get_msb() >= pcur->get_lsb())
 	    acc_off += (sb - pcur->get_lsb()) * acc_wid;
       else
-	    acc_off += (sb - pcur->get_msb()) * acc_wid;
+	    acc_off += (pcur->get_lsb() - sb) * acc_wid;
 
       if (prefix.empty()) {
 	    loff = acc_off;
@@ -115,7 +115,7 @@ bool prefix_to_slice(const std::vector<netrange_t>&dims,
 	    if (pcur->get_msb() >= pcur->get_lsb())
 		  acc_off += (*icur - pcur->get_lsb()) * acc_wid;
 	    else
-		  acc_off += (*icur - pcur->get_msb()) * acc_wid;
+		  acc_off += (pcur->get_lsb() - *icur) * acc_wid;
 
       } while (icur != prefix.begin());