diff --git a/elab_lval.cc b/elab_lval.cc
index 4f1859c43..f3858d86a 100644
--- a/elab_lval.cc
+++ b/elab_lval.cc
@@ -528,9 +528,8 @@ bool PEIdent::elaborate_lval_net_bit_(Design*des,
 				      bool need_const_idx,
 				      bool is_force) const
 {
-      list<long>prefix_indices;
-      bool rc = calculate_packed_indices_(des, scope, lv->sig(), prefix_indices);
-      if (!rc) return false;
+      NetNet*reg = lv->sig();
+      ivl_assert(*this, reg);
 
       const name_component_t&name_tail = path_.back();
       ivl_assert(*this, !name_tail.index.empty());
@@ -539,8 +538,66 @@ bool PEIdent::elaborate_lval_net_bit_(Design*des,
       ivl_assert(*this, index_tail.msb != 0);
       ivl_assert(*this, index_tail.lsb == 0);
 
-      NetNet*reg = lv->sig();
-      ivl_assert(*this, reg);
+	// First, check if packed prefix indices contain any variable expressions.
+	// Build the list of packed indices (excluding unpacked dimensions).
+      list<index_component_t> packed_index;
+      packed_index = name_tail.index;
+      for (size_t idx = 0 ; idx < reg->unpacked_dimensions() ; idx += 1)
+	    packed_index.pop_front();
+
+	// For multi-dimensional packed arrays, check if the prefix indices
+	// (all but the final) contain any non-constant expressions.
+      bool has_variable_prefix = false;
+      if (packed_index.size() > 1) {
+	    list<index_component_t>::const_iterator icur = packed_index.begin();
+	    for (size_t idx = 0 ; (idx+1) < packed_index.size() ; idx += 1, ++icur) {
+		  NetExpr*texpr = elab_and_eval(des, scope, icur->msb, -1, false);
+		  if (texpr == 0 || !dynamic_cast<NetEConst*>(texpr)) {
+			has_variable_prefix = true;
+		  }
+		  delete texpr;
+		  if (has_variable_prefix) break;
+	    }
+      }
+
+	// If prefix indices are variable, handle using expression-based path.
+      if (has_variable_prefix) {
+	    if (need_const_idx) {
+		  cerr << get_fileline() << ": error: '" << reg->name()
+		       << "' index must be a constant in this context."
+		       << endl;
+		  des->errors += 1;
+		  return false;
+	    }
+
+	    if ((reg->type()==NetNet::UNRESOLVED_WIRE) && !is_force) {
+		  ivl_assert(*this, reg->coerced_to_uwire());
+		  report_mixed_assignment_conflict_("bit select");
+		  des->errors += 1;
+		  return false;
+	    }
+
+	      // Use collapse_array_exprs to compute the bit offset as an expression.
+	    NetExpr*base_expr = collapse_array_exprs(des, scope, this, reg, packed_index);
+	    if (base_expr == 0) {
+		  des->errors += 1;
+		  return false;
+	    }
+
+	    if (debug_elaborate) {
+		  cerr << get_fileline() << ": PEIdent::elaborate_lval_net_bit_: "
+		       << "Variable packed prefix, base_expr=" << *base_expr
+		       << endl;
+	    }
+
+	    lv->set_part(base_expr, 1);
+	    return true;
+      }
+
+	// All prefix indices are constant. Use the existing code path.
+      list<long>prefix_indices;
+      bool rc = calculate_packed_indices_(des, scope, reg, prefix_indices);
+      if (!rc) return false;
 
 	// Bit selects have a single select expression. Evaluate the
 	// constant value and treat it as a part select with a bit
diff --git a/ivtest/ivltests/br_gh521.v b/ivtest/ivltests/br_gh521.v
new file mode 100644
index 000000000..fb8bf40e2
--- /dev/null
+++ b/ivtest/ivltests/br_gh521.v
@@ -0,0 +1,19 @@
+// Test for GitHub issue #521
+// Loop index should be allowed in outer dimension of multi-dimensional packed arrays
+module test;
+  logic [3:0][3:0] a;
+
+  initial begin
+    a = 0;
+    for (int i=0; i<4; i++)
+      a[i][3] = 1;
+
+    // Each 4-bit sub-array has bit 3 set to 1, so each nibble is 0x8
+    if (a !== 16'h8888) begin
+      $display("FAILED: a = %h, expected 8888", a);
+      $finish;
+    end
+
+    $display("PASSED");
+  end
+endmodule
diff --git a/ivtest/regress-sv.list b/ivtest/regress-sv.list
index ded8b92a1..76846780a 100644
--- a/ivtest/regress-sv.list
+++ b/ivtest/regress-sv.list
@@ -212,6 +212,7 @@ br_gh477		normal,-g2009		ivltests
 br_gh478		normal,-g2009		ivltests
 br_gh498		normal,-g2009		ivltests
 br_gh508a		normal,-g2009		ivltests
+br_gh521		normal,-g2012		ivltests
 br_gh527		normal,-g2009		ivltests
 br_gh530		CO,-g2009		ivltests
 br_gh540		normal,-g2009		ivltests