diff --git a/cpp/BUILD b/cpp/BUILD
index 1776e802..d8f2251e 100644
--- a/cpp/BUILD
+++ b/cpp/BUILD
@@ -53,6 +53,22 @@ cc_test(
     testonly = True,
 )
 
+# Unit test for code area library
+cc_test(
+    name = "codearea_test",
+    size = "small",
+    srcs = ["codearea_test.cc"],
+    copts = [
+        "-pthread",
+        "-Iexternal/gtest/include",
+    ],
+    linkopts = ["-pthread"],
+    deps = [
+        ":codearea",
+        "@gtest//:main",
+    ],
+)
+
 # Example binary for open location codes
 cc_binary(
     name = "openlocationcode_example",
diff --git a/cpp/codearea.cc b/cpp/codearea.cc
index 4d0c65ee..f40320b4 100644
--- a/cpp/codearea.cc
+++ b/cpp/codearea.cc
@@ -4,8 +4,11 @@
 
 namespace openlocationcode {
 
+namespace {
 const double kLatitudeMaxDegrees = 90;
 const double kLongitudeMaxDegrees = 180;
+const CodeArea kInvalidCodeArea(0.0, 0.0, 0.0, 0.0, 0);
+}  // anonymous namespace
 
 CodeArea::CodeArea(double latitude_lo, double longitude_lo, double latitude_hi,
                    double longitude_hi, size_t code_length) {
@@ -36,4 +39,13 @@ LatLng CodeArea::GetCenter() const {
   return center;
 }
 
+bool CodeArea::IsValid() const {
+  return code_length_ > 0 && latitude_lo_ < latitude_hi_ &&
+         longitude_lo_ < longitude_hi_ && latitude_lo_ >= -90.0 &&
+         latitude_hi_ <= 90.0 && longitude_lo_ >= -180.0 &&
+         longitude_hi_ <= 180.0;
+}
+
+const CodeArea& CodeArea::InvalidCodeArea() { return kInvalidCodeArea; }
+
 }  // namespace openlocationcode
diff --git a/cpp/codearea.h b/cpp/codearea.h
index 78abca18..d5e7ab5f 100644
--- a/cpp/codearea.h
+++ b/cpp/codearea.h
@@ -20,6 +20,10 @@ class CodeArea {
   double GetLongitudeHi() const;
   size_t GetCodeLength() const;
   LatLng GetCenter() const;
+  // Returns whether or not this area was a valid decode result.
+  bool IsValid() const;
+  // Returns an invalid CodeArea object.
+  static const CodeArea& InvalidCodeArea();
 
  private:
   double latitude_lo_;
diff --git a/cpp/codearea_test.cc b/cpp/codearea_test.cc
new file mode 100644
index 00000000..cc0b8290
--- /dev/null
+++ b/cpp/codearea_test.cc
@@ -0,0 +1,59 @@
+#include "codearea.h"
+#include "gtest/gtest.h"
+
+namespace openlocationcode {
+namespace {
+
+TEST(CodeAreaChecks, Accessors) {
+  const CodeArea area(1.0, 2.0, 3.0, 4.0, 6);
+  // Check accessor methods return what we expect.
+  EXPECT_EQ(area.GetLatitudeLo(), 1.0);
+  EXPECT_EQ(area.GetLongitudeLo(), 2.0);
+  EXPECT_EQ(area.GetLatitudeHi(), 3.0);
+  EXPECT_EQ(area.GetLongitudeHi(), 4.0);
+  EXPECT_EQ(area.GetCodeLength(), 6);
+}
+
+TEST(CodeAreaChecks, GetCenter) {
+  // Simple case.
+  const CodeArea area1(0.0, 0.0, 1.0, 2.0, 8);
+  EXPECT_EQ(area1.GetCenter().latitude, 0.5);
+  EXPECT_EQ(area1.GetCenter().longitude, 1.0);
+  // Negative latitudes & longitudes.
+  const CodeArea area2(-10.0, -30.0, -24.0, -84.0, 4);
+  EXPECT_EQ(area2.GetCenter().latitude, -17.0);
+  EXPECT_EQ(area2.GetCenter().longitude, -57.0);
+  // Latitude & longitude ranges crossing zero.
+  const CodeArea area3(-30.0, -17.0, 5.0, 21.0, 4);
+  EXPECT_EQ(area3.GetCenter().latitude, -12.5);
+  EXPECT_EQ(area3.GetCenter().longitude, 2.0);
+  // Zero-sized area (not strictly valid, but center is still well-defined).
+  const CodeArea area4(-65.0, 117.0, -65.0, 117.0, 2);
+  EXPECT_EQ(area4.GetCenter().latitude, -65.0);
+  EXPECT_EQ(area4.GetCenter().longitude, 117.0);
+}
+
+TEST(CodeAreaChecks, IsValid) {
+  // All zeroes: invalid.
+  EXPECT_FALSE(CodeArea(0.0, 0.0, 0.0, 0.0, 0).IsValid());
+  // Whole-world area: valid.
+  EXPECT_TRUE(CodeArea(-90.0, -180.0, 90.0, 180.0, 1).IsValid());
+  // Typical area: valid.
+  EXPECT_TRUE(CodeArea(-1.0, -1.0, 1.0, 1.0, 10).IsValid());
+  // Zero code length: invalid.
+  EXPECT_FALSE(CodeArea(-1.0, -1.0, 1.0, 1.0, 0).IsValid());
+  // Low latitude >= high latitude: invalid.
+  EXPECT_FALSE(CodeArea(1.0, -1.0, 1.0, 1.0, 10).IsValid());
+  EXPECT_FALSE(CodeArea(2.0, -1.0, 1.0, 1.0, 10).IsValid());
+  // Low longitude >= high longitude: invalid.
+  EXPECT_FALSE(CodeArea(-1.0, 1.0, 1.0, 1.0, 10).IsValid());
+  EXPECT_FALSE(CodeArea(-1.0, 2.0, 1.0, 1.0, 10).IsValid());
+}
+
+TEST(CodeAreaChecks, InvalidCodeArea) {
+  // CodeArea::InvalideCodeArea() must return an invalid code area, obviously.
+  EXPECT_FALSE(CodeArea::InvalidCodeArea().IsValid());
+}
+
+}  // namespace
+}  // namespace openlocationcode
diff --git a/cpp/openlocationcode.cc b/cpp/openlocationcode.cc
index b763ff1c..929f0fcd 100644
--- a/cpp/openlocationcode.cc
+++ b/cpp/openlocationcode.cc
@@ -209,8 +209,13 @@ CodeArea Decode(const std::string &code) {
   // Define the place value for the most significant pair.
   int pv = pow(internal::kEncodingBase, internal::kPairCodeLength / 2 - 1);
   for (size_t i = 0; i < digits - 1; i += 2) {
-    normal_lat += get_alphabet_position(clean_code[i]) * pv;
-    normal_lng += get_alphabet_position(clean_code[i + 1]) * pv;
+    const int lat_dval = get_alphabet_position(clean_code[i]);
+    const int lng_dval = get_alphabet_position(clean_code[i + 1]);
+    if (lat_dval < 0 || lng_dval < 0) {
+      return CodeArea::InvalidCodeArea();
+    }
+    normal_lat += lat_dval * pv;
+    normal_lng += lng_dval * pv;
     if (i < digits - 2) {
       pv /= internal::kEncodingBase;
     }
@@ -227,6 +232,9 @@ CodeArea Decode(const std::string &code) {
     digits = std::min(internal::kMaximumDigitCount, clean_code.size());
     for (size_t i = internal::kPairCodeLength; i < digits; i++) {
       int dval = get_alphabet_position(clean_code[i]);
+      if (dval < 0) {
+        return CodeArea::InvalidCodeArea();
+      }
       int row = dval / internal::kGridColumns;
       int col = dval % internal::kGridColumns;
       extra_lat += row * row_pv;
diff --git a/cpp/openlocationcode_test.cc b/cpp/openlocationcode_test.cc
index 6e6b01e9..9780000c 100644
--- a/cpp/openlocationcode_test.cc
+++ b/cpp/openlocationcode_test.cc
@@ -170,6 +170,15 @@ TEST_P(EncodingChecks, Encode) {
 INSTANTIATE_TEST_CASE_P(OLC_Tests, EncodingChecks,
                         ::testing::ValuesIn(GetEncodingDataFromCsv()));
 
+TEST(DecodingChecks, DecodeOutOfRange) {
+  // Leading longitude digit "W" is out of range.
+  EXPECT_FALSE(Decode(std::string("9W4Q0000+")).IsValid());
+  // Leading latitude digit "F" is out of range.
+  EXPECT_FALSE(Decode(std::string("F2+")).IsValid());
+  // Invalid digits in post-separator portion.
+  EXPECT_FALSE(Decode(std::string("7Q7Q7Q7Q+5Z")).IsValid());
+}
+
 struct ValidityTestData {
   std::string code;
   bool is_valid;