diff --git a/lib/elixir/src/elixir_bitstring.erl b/lib/elixir/src/elixir_bitstring.erl
index 43354ad75c..8daa0acc4a 100644
--- a/lib/elixir/src/elixir_bitstring.erl
+++ b/lib/elixir/src/elixir_bitstring.erl
@@ -226,9 +226,9 @@ build_spec(Meta, Size, Unit, Type, Endianess, Sign, Spec, E) when Type == intege
       add_spec(Type, add_spec(Endianess, add_spec(Sign, Spec)))
   end.
 
-number_size(default, _) -> default;
-number_size(Size, default) -> Size;
-number_size(Size, Unit) -> Size * Unit.
+number_size(Size, default) when is_integer(Size) -> Size;
+number_size(Size, Unit) when is_integer(Size) -> Size * Unit;
+number_size(Size, _) -> Size.
 
 add_spec(default, Spec) -> Spec;
 add_spec(Key, Spec) -> [{Key, [], []} | Spec].
diff --git a/lib/elixir/test/elixir/kernel/expansion_test.exs b/lib/elixir/test/elixir/kernel/expansion_test.exs
index c9385909c7..ca3a51873a 100644
--- a/lib/elixir/test/elixir/kernel/expansion_test.exs
+++ b/lib/elixir/test/elixir/kernel/expansion_test.exs
@@ -1078,6 +1078,12 @@ defmodule Kernel.ExpansionTest do
              quote do: (:"Elixir.Kernel.ExpansionTarget"; <<x()::integer()-size(17)>>)
     end
 
+    test "supports dynamic size" do
+      import Kernel, except: [-: 2]
+      assert expand(quote do: (var = 1; <<x::size(var)-unit(8)>>)) ==
+             quote do: (var = 1; <<x()::integer()-unit(8)-size(var)>>)
+    end
+
     test "raises on size or unit for literal bitstrings" do
       assert_raise CompileError, ~r"literal <<>> in bitstring supports only type specifiers", fn ->
         expand(quote do: << <<"foo">>::32 >>)