diff --git a/lib/ex_junk.ex b/lib/ex_junk.ex
index 31ecbff..f2b7a47 100644
--- a/lib/ex_junk.ex
+++ b/lib/ex_junk.ex
@@ -35,11 +35,14 @@ defmodule Junk do
     junk_for_type(junk_type, merged_opts)
   end
 
+  defp junk_for_type(:"Elixir.String", %{prefix: nil} = opts), do: random_string(opts)
+  defp junk_for_type(:"Elixir.String", %{prefix: ""} = opts), do: random_string(opts)
   defp junk_for_type(:"Elixir.String", opts) do
-    string = random_bytes(opts)
-    |> Base.url_encode64
-    opts.prefix <> "-" <> string
+    opts
+    |> random_string
+    |> String.replace_prefix("", opts.prefix <> "-")
   end
+
   defp junk_for_type(:"Elixir.Integer", opts) do
     min = :math.pow(10, opts.size-1) |> trunc
     max = :math.pow(10, opts.size)-1 |> trunc
@@ -49,4 +52,8 @@ defmodule Junk do
   defp random_bytes(%{rand_mod: rand_mod, byte_size: b_size}) do
     rand_mod.random_bytes.(b_size)
   end
+
+  defp random_string(opts) do
+    opts |> random_bytes |> Base.url_encode64
+  end
 end
diff --git a/test/ex_junk_test.exs b/test/ex_junk_test.exs
index ce863aa..6a048bc 100644
--- a/test/ex_junk_test.exs
+++ b/test/ex_junk_test.exs
@@ -28,6 +28,14 @@ defmodule JunkTest do
     assert String.starts_with?(output, "prefix-")
   end
 
+  test "String doesn't start with dash if prefix empty" do
+    output_with_blank_prefix = Junk.junk(String)
+    output_with_nil_prefix = Junk.junk(String, prefix: nil)
+
+    refute String.starts_with?(output_with_blank_prefix, "-")
+    refute String.starts_with?(output_with_nil_prefix, "-")
+  end
+
   test "returns unique Integers" do
     assert Junk.junk(Integer) |> is_integer == true
     assert Junk.junk(Integer) != Junk.junk(Integer)