diff --git a/include/rapidjson/internal/strtod.h b/include/rapidjson/internal/strtod.h
index 01463fe..46248bc 100644
--- a/include/rapidjson/internal/strtod.h
+++ b/include/rapidjson/internal/strtod.h
@@ -122,22 +122,6 @@ public:
         return *this;
     }
 
-    BigInteger& operator+=(const BigInteger& rhs) {
-        size_t count = count_ > rhs.count_ ? count_ : rhs.count_;
-        bool carry = false;
-        for (size_t i = 0; i < count; i++) {
-            bool outCarry;
-            digits_[i] = FullAdd(i < count_ ? digits_[i] : 0, i < rhs.count_ ? rhs.digits_[i] : 0, carry, &outCarry);
-            carry = outCarry;
-        }
-        count_ = count;
-
-        if (carry)
-            PushBack(1);
-
-        return *this;
-    }
-
     BigInteger& operator*=(uint64_t u) {
         if (u == 0) return *this = 0;
         if (u == 1) return *this;
diff --git a/test/unittest/strtodtest.cpp b/test/unittest/strtodtest.cpp
index d0319c2..ae56412 100644
--- a/test/unittest/strtodtest.cpp
+++ b/test/unittest/strtodtest.cpp
@@ -66,30 +66,6 @@ TEST(Strtod, BigInteger_AddUint64) {
     EXPECT_TRUE(BIGINTEGER_LITERAL("36893488147419103231") == b);
 }
 
-TEST(Strtod, BigInteger_Add) {
-    BigInteger a = kZero;
-    a += kZero;
-    EXPECT_TRUE(kZero == a);
-
-    a += kOne;
-    EXPECT_TRUE(kOne == a);
-
-    a += kOne;
-    EXPECT_TRUE(BigInteger(2) == a);
-
-    a = kUint64Max;
-    a += kOne;
-    EXPECT_TRUE(kTwo64 == a);
-
-    a = kOne;
-    a += kTwo64;
-    EXPECT_TRUE(BIGINTEGER_LITERAL("18446744073709551617") == a);
-    
-    a = kTwo64;
-    a += kOne;
-    EXPECT_TRUE(BIGINTEGER_LITERAL("18446744073709551617") == a);
-}
-
 TEST(Strtod, BigInteger_MultiplyUint64) {
     BigInteger a = kZero;
     a *= static_cast <uint64_t>(0);