diff --git a/AGENTS.md b/AGENTS.md
index 0b846ab..ee5a94c 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -78,7 +78,7 @@ ItaxCode.decode(tax_code) #=> Hash:
 #   code:       String,          # the input code, upcased
 #   gender:     "M" | "F",
 #   birthdate:  String,          # "YYYY-MM-DD"
-#   birthplace: {                # nil if not found in either CSV
+#   birthplace: {                # raises InvalidTaxCodeError if not found in either CSV
 #     code:       String,        # e.g. "F205"
 #     province:   String,        # e.g. "MI"
 #     name:       String,        # e.g. "MILANO"
diff --git a/lib/itax_code/parser.rb b/lib/itax_code/parser.rb
index 97c1ee0..e110025 100644
--- a/lib/itax_code/parser.rb
+++ b/lib/itax_code/parser.rb
@@ -92,10 +92,12 @@ def birthplace(src = utils.cities, stop: false)
         place = src.find { |item| item["code"] == birthplace_code && in_dates?(item) }
 
         if place.nil?
-          birthplace(utils.countries, stop: true) unless stop
-        else
-          place.to_h.transform_keys(&:to_sym)
+          return birthplace(utils.countries, stop: true) unless stop
+
+          raise InvalidTaxCodeError
         end
+
+        place.to_h.transform_keys(&:to_sym)
       end
 
       def birthplace_code
diff --git a/test/itax_code/encoder_test.rb b/test/itax_code/encoder_test.rb
index 2e30ff5..2d4be82 100644
--- a/test/itax_code/encoder_test.rb
+++ b/test/itax_code/encoder_test.rb
@@ -54,12 +54,64 @@ class EncoderTest < Minitest::Test
                    ).encode
     end
 
+    test "#encode for female gender applies +40 day offset" do
+      assert_equal "RSSMRA80A41F205B",
+                   Encoder.new(
+                     surname: "Rossi",
+                     name: "Mario",
+                     gender: "F",
+                     birthdate: "1980-01-01",
+                     birthplace: "Milano"
+                   ).encode
+    end
+
     test "#encode raises MissingDataError on missing data" do
       assert_raises Encoder::MissingDataError do
         Encoder.new.encode
       end
     end
 
+    test "#encode raises MissingDataError on missing surname" do
+      assert_raises(Encoder::MissingDataError) do
+        Encoder.new(
+          surname: nil, name: "Mario", gender: "M", birthdate: "1980-01-01", birthplace: "Milano"
+        )
+      end
+    end
+
+    test "#encode raises MissingDataError on missing name" do
+      assert_raises(Encoder::MissingDataError) do
+        Encoder.new(
+          surname: "Rossi", name: nil, gender: "M", birthdate: "1980-01-01", birthplace: "Milano"
+        )
+      end
+    end
+
+    test "#encode raises MissingDataError on missing gender" do
+      assert_raises(Encoder::MissingDataError) do
+        Encoder.new(
+          surname: "Rossi", name: "Mario", gender: nil,
+          birthdate: "1980-01-01", birthplace: "Milano"
+        )
+      end
+    end
+
+    test "#encode raises MissingDataError on missing birthdate" do
+      assert_raises(Encoder::MissingDataError) do
+        Encoder.new(
+          surname: "Rossi", name: "Mario", gender: "M", birthdate: nil, birthplace: "Milano"
+        )
+      end
+    end
+
+    test "#encode raises MissingDataError on missing birthplace field" do
+      assert_raises(Encoder::MissingDataError) do
+        Encoder.new(
+          surname: "Rossi", name: "Mario", gender: "M", birthdate: "1980-01-01", birthplace: nil
+        )
+      end
+    end
+
     test "#encode raises MissingDataError on missing birthplace" do
       assert_raises Encoder::MissingDataError do
         Encoder.new(
diff --git a/test/itax_code/parser_test.rb b/test/itax_code/parser_test.rb
index ba12176..754cfcd 100644
--- a/test/itax_code/parser_test.rb
+++ b/test/itax_code/parser_test.rb
@@ -29,9 +29,8 @@ class ParserTest < Minitest::Test
       end
     end
 
-    # FIXME: This behaviour needs to be fixed, maybe by raising an InvalidTaxCodeError.
-    test "returns nil birthplace when the lookup on both cities and countries fails" do
-      assert_nil klass.new("BRRDRN70M41ZXXXE").decode[:birthplace]
+    test "raises InvalidTaxCodeError when birthplace code is not found in cities or countries" do
+      assert_raises(klass::InvalidTaxCodeError) { klass.new("BRRDRN70M41ZXXXE").decode }
     end
 
     test "raises NoTaxCodeError with an empty tax code" do
@@ -62,6 +61,45 @@ class ParserTest < Minitest::Test
       end
     end
 
+    test "raises InvalidTaxCodeError when birthdate falls outside city validity window" do
+      # Stub cities to expose a single F205 entry whose window excludes birthdate 1980-01-01
+      stub_city = { "code" => "F205", "province" => "MI", "name" => "MILANO",
+                    "created_on" => "1960-01-01", "deleted_on" => "1975-12-31" }
+      Utils.stubs(:cities).returns([stub_city])
+      Utils.stubs(:countries).returns([])
+      # in_dates? returns false (1980-01-01 > deleted_on) → F205 not matched in cities
+      # F205 not in countries either → raises InvalidTaxCodeError
+      assert_raises(klass::InvalidTaxCodeError) { klass.new("RSSMRA80A01F205X").decode }
+    end
+
+    test "decodes a birthdate in the current year" do
+      Timecop.freeze(Date.parse("2026-06-01")) do
+        # year "26" → 20+26=2026, 2026 <= 2026 → 2026 (not 1926)
+        assert_equal "2026-01-01", klass.new("RSSMRA26A01L219E").decode[:birthdate]
+      end
+    end
+
+    test "decodes a birthdate as 1927 when current year is 2026 and year code is 27" do
+      Timecop.freeze(Date.parse("2026-06-01")) do
+        # year "27" → 20+27=2027, 2027 > 2026 → 2027-100 = 1927
+        assert_equal "1927-01-01", klass.new("RSSMRA27A01L219F").decode[:birthdate]
+      end
+    end
+
+    test "decodes day 31 as male gender with day 31" do
+      result = klass.new("RSSMRA80A31L219P").decode
+
+      assert_equal "M",          result[:gender]
+      assert_equal "1980-01-31", result[:birthdate]
+    end
+
+    test "decodes day 41 as female gender with day 1" do
+      result = klass.new("RSSMRA80A41F205B").decode
+
+      assert_equal "F",          result[:gender]
+      assert_equal "1980-01-01", result[:birthdate]
+    end
+
     private
 
       def klass
diff --git a/test/itax_code_test.rb b/test/itax_code_test.rb
index 63b285e..4045501 100644
--- a/test/itax_code_test.rb
+++ b/test/itax_code_test.rb
@@ -27,6 +27,33 @@ class ItaxCodeTest < Minitest::Test
     refute klass.valid?("WRONG")
   end
 
+  test "encode/decode round-trip for male" do
+    result = klass.decode(klass.encode(surname: "Rossi", name: "Mario", gender: "M",
+                                       birthdate: "1980-01-01", birthplace: "Milano"))
+
+    assert_equal "M",          result[:gender]
+    assert_equal "1980-01-01", result[:birthdate]
+    assert_equal "F205",       result[:birthplace][:code]
+  end
+
+  test "encode/decode round-trip for female (day offset)" do
+    result = klass.decode(klass.encode(surname: "Rossi", name: "Mario", gender: "F",
+                                       birthdate: "1980-01-01", birthplace: "Milano"))
+
+    assert_equal "F",          result[:gender]
+    assert_equal "1980-01-01", result[:birthdate]
+    assert_equal "F205",       result[:birthplace][:code]
+  end
+
+  test "encode/decode round-trip for foreign birthplace" do
+    result = klass.decode(klass.encode(surname: "Berardi", name: "Adriana", gender: "F",
+                                       birthdate: "1970-08-01", birthplace: "Brasile"))
+
+    assert_equal "F",          result[:gender]
+    assert_equal "1970-08-01", result[:birthdate]
+    assert_equal "Z602",       result[:birthplace][:code]
+  end
+
   private
 
     def klass