diff --git a/.github/workflows/rspec.yml b/.github/workflows/rspec.yml
index bcd017e..f7aaca9 100644
--- a/.github/workflows/rspec.yml
+++ b/.github/workflows/rspec.yml
@@ -9,6 +9,8 @@ jobs:
       fail-fast: false
       matrix:
         ruby-version: ['2.7', '3.0', '3.1', '3.2', 'head']
+    env:
+      NO_STEEP: "Because we need to test on versions that are older than steep supports"
 
     steps:
       - uses: actions/checkout@v3
diff --git a/.github/workflows/type_check.yml b/.github/workflows/type_check.yml
new file mode 100644
index 0000000..e15f94f
--- /dev/null
+++ b/.github/workflows/type_check.yml
@@ -0,0 +1,18 @@
+name: Steep Type Checking
+
+on: push
+
+jobs:
+  check:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v3
+
+      - name: Set up ruby
+        uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: 3.3
+          bundler-cache: true
+
+      - name: Run Steep
+        run: bundle exec steep check
diff --git a/Gemfile b/Gemfile
index b4e2a20..026f34f 100644
--- a/Gemfile
+++ b/Gemfile
@@ -1,3 +1,7 @@
 source "https://rubygems.org"
 
 gemspec
+
+unless ENV["NO_STEEP"]
+  gem "steep", "~> 1.6.0"
+end
diff --git a/Steepfile b/Steepfile
new file mode 100644
index 0000000..a9bffad
--- /dev/null
+++ b/Steepfile
@@ -0,0 +1,17 @@
+D = Steep::Diagnostic
+
+target :lib do
+  signature "sig"
+  check "lib"
+  configure_code_diagnostics(D::Ruby.default)
+  library "pathname"
+  library "date"
+end
+
+target :test do
+  signature "sig"
+  check "test"
+  configure_code_diagnostics(D::Ruby.default)
+  library "pathname"
+  library "date"
+end
diff --git a/lib/environment_helpers.rb b/lib/environment_helpers.rb
index 35d6683..a0f47af 100644
--- a/lib/environment_helpers.rb
+++ b/lib/environment_helpers.rb
@@ -2,14 +2,14 @@
 # And we're compatible back to 2.6
 require "set" # rubocop:disable Lint/RedundantRequireStatement
 
-require_relative "./environment_helpers/access_helpers"
-require_relative "./environment_helpers/string_helpers"
-require_relative "./environment_helpers/boolean_helpers"
-require_relative "./environment_helpers/range_helpers"
-require_relative "./environment_helpers/numeric_helpers"
-require_relative "./environment_helpers/file_helpers"
-require_relative "./environment_helpers/datetime_helpers"
-require_relative "./environment_helpers/enumerable_helpers"
+require_relative "environment_helpers/access_helpers"
+require_relative "environment_helpers/string_helpers"
+require_relative "environment_helpers/boolean_helpers"
+require_relative "environment_helpers/range_helpers"
+require_relative "environment_helpers/numeric_helpers"
+require_relative "environment_helpers/file_helpers"
+require_relative "environment_helpers/datetime_helpers"
+require_relative "environment_helpers/enumerable_helpers"
 
 module EnvironmentHelpers
   Error = Class.new(::StandardError)
diff --git a/lib/environment_helpers/enumerable_helpers.rb b/lib/environment_helpers/enumerable_helpers.rb
index 4458fa4..1562802 100644
--- a/lib/environment_helpers/enumerable_helpers.rb
+++ b/lib/environment_helpers/enumerable_helpers.rb
@@ -2,6 +2,18 @@ module EnvironmentHelpers
   module EnumerableHelpers
     VALID_TYPES = %i[strings symbols integers]
 
+    TYPE_HANDLERS = {
+      integers: :to_i,
+      strings: :to_s,
+      symbols: :to_sym
+    }
+
+    TYPE_MAP = {
+      integers: Integer,
+      strings: String,
+      symbols: Symbol
+    }
+
     def array(key, of: :strings, delimiter: ",", default: nil, required: false)
       check_default_type(:array, default, Array)
       check_valid_data_type!(of)
@@ -22,23 +34,12 @@ def check_valid_data_type!(type)
     end
 
     def check_default_data_types!(default, type)
+      return if default.nil?
       invalid = Array(default).reject { |val| val.is_a? TYPE_MAP[type] }
 
       unless invalid.empty?
         fail(BadDefault, "Default array contains values not of type `#{type}': #{invalid.join(", ")}")
       end
     end
-
-    TYPE_HANDLERS = {
-      integers: :to_i,
-      strings: :to_s,
-      symbols: :to_sym
-    }
-
-    TYPE_MAP = {
-      integers: Integer,
-      strings: String,
-      symbols: Symbol
-    }
   end
 end
diff --git a/lib/environment_helpers/range_helpers.rb b/lib/environment_helpers/range_helpers.rb
index 311c535..2019b3b 100644
--- a/lib/environment_helpers/range_helpers.rb
+++ b/lib/environment_helpers/range_helpers.rb
@@ -6,7 +6,7 @@ def integer_range(name, default: nil, required: false)
       check_range_endpoint(:integer_range, default.end) if default
 
       text = fetch_value(name, required: required)
-      range = parse_range_from(text)
+      range = text ? parse_range_from(text) : nil
       return range if range
       return default unless required
       fail(InvalidRangeText, "Required Integer Range environment variable #{name} had inappropriate content '#{text}'")
diff --git a/lib/environment_helpers/string_helpers.rb b/lib/environment_helpers/string_helpers.rb
index c4f9d8f..8f94a2b 100644
--- a/lib/environment_helpers/string_helpers.rb
+++ b/lib/environment_helpers/string_helpers.rb
@@ -6,7 +6,7 @@ def string(name, default: nil, required: false)
 
     def symbol(name, default: nil, required: false)
       check_default_type(:symbol, default, Symbol)
-      string(name, default: default, required: required)&.to_sym
+      string(name, default: default&.to_s, required: required)&.to_sym
     end
   end
 end
diff --git a/sig/environment_helpers.rbs b/sig/environment_helpers.rbs
new file mode 100644
index 0000000..e98fccb
--- /dev/null
+++ b/sig/environment_helpers.rbs
@@ -0,0 +1,96 @@
+module EnvironmentHelpers
+  VERSION: String
+
+  class Error < StandardError
+  end
+
+  class MissingVariableError < Error
+  end
+
+  class BadDefault < Error
+  end
+
+  class BadFormat < Error
+  end
+
+  class InvalidType < Error
+  end
+
+  class InvalidValue < Error
+  end
+
+  class InvalidBooleanText < InvalidValue
+  end
+
+  class InvalidRangeText < InvalidValue
+  end
+
+  class InvalidIntegerText < InvalidValue
+  end
+
+  class InvalidDateText < InvalidValue
+  end
+
+  class InvalidDateTimeText < InvalidValue
+  end
+
+  module AccessHelpers
+    # (Actually provided by ENV, which these are all extended onto)
+    def fetch: (String name) -> String?
+             | (String name, String? default) -> String?
+
+    private def fetch_value: (String name, required: bool) -> String?
+    private def check_default_type: (String | Symbol context, untyped value, *Class types) -> void
+    private def check_default_value: (String | Symbol context, untyped value, allow: Enumerable[untyped]) -> void
+  end
+
+  module FileHelpers : AccessHelpers
+    def file_path: (String name, default: String?, required: bool) -> Pathname?
+  end
+
+  module DatetimeHelpers : AccessHelpers
+    type date_time_result = DateTime? | Time?
+    def date: (String name, format: String, default: Date?, required: bool) -> Date?
+    def date_time: (String name, format: String | Symbol, default: date_time_result, required: bool) -> date_time_result
+    private def parse_date_from: (String? text, format: String) -> Date?
+    private def parse_date_time_from: (String? text, format: String | Symbol) -> DateTime?
+    private def iso8601_date_time: (String) -> DateTime?
+    private def unix_date_time: (String) -> DateTime?
+    private def strptime_date_time: (String, format: String) -> DateTime?
+  end
+
+  module NumericHelpers  : AccessHelpers
+    def integer: (String name, default: Integer?, required: bool) -> Integer?
+  end
+
+  module RangeHelpers : AccessHelpers
+    def integer_range: (String name, default: Range[Integer]?, required: bool) -> Range[Integer]?
+    private def check_range_endpoint: (String | Symbol context, untyped value) -> void
+    private def parse_range_bound_from: (String?) -> Integer?
+    private def parse_range_from: (String) -> Range[Integer]?
+  end
+
+  module BooleanHelpers : AccessHelpers
+    TRUTHY_STRINGS: Set[String]
+    FALSEY_STRINGS: Set[String]
+    BOOLEAN_VALUES: Set[bool]
+    def boolean: (String name, default: bool?, required: bool) -> boolish
+    private def truthy_text?: (String?) -> boolish
+    private def falsey_text?: (String?) -> boolish
+  end
+
+  module EnumerableHelpers : AccessHelpers
+    VALID_TYPES: Array[Symbol]
+    TYPE_HANDLERS: Hash[Symbol, Symbol]
+    TYPE_MAP: Hash[Symbol, Class]
+    type arrayable = Integer | String | Symbol
+    def array: (String key, of: Symbol, delimiter: String, default: Array[arrayable]?, required: bool) -> Array[arrayable]?
+    private def check_valid_data_type!: (Symbol) -> void
+    private def check_default_data_types!: (Array[arrayable]?, Symbol) -> void
+  end
+
+  module StringHelpers : AccessHelpers
+    def string: (String name, default: String?, required: bool) -> String?
+    def symbol: (String name, default: Symbol?, required: bool) -> Symbol?
+  end
+end
diff --git a/spec/environment_helpers/enumerable_helpers_spec.rb b/spec/environment_helpers/enumerable_helpers_spec.rb
index 72ae080..2e6a620 100644
--- a/spec/environment_helpers/enumerable_helpers_spec.rb
+++ b/spec/environment_helpers/enumerable_helpers_spec.rb
@@ -7,6 +7,12 @@
     subject(:array) { env.array(env_var, **params) }
 
     with_env "FOO" => "a,bc,d"
+    without_env "FOOBAR"
+
+    context "when the value is not present" do
+      let(:env_var) { "FOOBAR" }
+      it { should be_nil }
+    end
 
     describe "parameters" do
       context "when passed an `of' param" do
diff --git a/spec/environment_helpers/string_helpers_spec.rb b/spec/environment_helpers/string_helpers_spec.rb
index b6122a5..b61b1c8 100644
--- a/spec/environment_helpers/string_helpers_spec.rb
+++ b/spec/environment_helpers/string_helpers_spec.rb
@@ -79,7 +79,7 @@
       end
 
       context "when the env value is not set" do
-        before { expect(ENV["FOO"]).to be_nil }
+        without_env "FOO"
         it { is_expected.to be_nil }
       end
     end
diff --git a/spec/support/with_environment.rb b/spec/support/with_environment.rb
index c4058c8..e7b8487 100644
--- a/spec/support/with_environment.rb
+++ b/spec/support/with_environment.rb
@@ -10,6 +10,18 @@ def with_env(overrides)
       end
     end
   end
+
+  def without_env(*keys)
+    around(:each) do |ex|
+      orig_env = ENV.to_h
+      keys.each { |key| ENV.delete(key) }
+      begin
+        ex.run
+      ensure
+        ENV.replace(orig_env)
+      end
+    end
+  end
 end
 
 RSpec.configure do |config|