Verilator Internal Error for shift by undef value

Verilator 4a58e85 creates the following error:
%Error: Internal Error: ../V3Number.cpp:521: toUInt with 4-state 4'bxxxx

module issue_039(a, y);
  input [3:0] a;
  output [3:0] y;
  assign y = a >>> 4'bx;
endmodule

History:
2014-05-04 Reported as Issue #760
2014-05-04 Fixed in GIT commit 621c515