diff --git a/include/rapidjson/allocators.h b/include/rapidjson/allocators.h
index 05e3b0c..8cde8f4 100644
--- a/include/rapidjson/allocators.h
+++ b/include/rapidjson/allocators.h
@@ -194,6 +194,9 @@ public:
         if (newSize == 0)
             return NULL;
 
+        originalSize = RAPIDJSON_ALIGN(originalSize);
+        newSize = RAPIDJSON_ALIGN(newSize);
+
         // Do not shrink if new size is smaller than original
         if (originalSize >= newSize)
             return originalPtr;
@@ -201,7 +204,6 @@ public:
         // Simply expand it if it is the last allocation and there is sufficient space
         if (originalPtr == reinterpret_cast<char *>(chunkHead_) + RAPIDJSON_ALIGN(sizeof(ChunkHeader)) + chunkHead_->size - originalSize) {
             size_t increment = static_cast<size_t>(newSize - originalSize);
-            increment = RAPIDJSON_ALIGN(increment);
             if (chunkHead_->size + increment <= chunkHead_->capacity) {
                 chunkHead_->size += increment;
                 return originalPtr;
diff --git a/test/unittest/allocatorstest.cpp b/test/unittest/allocatorstest.cpp
index f70e672..a5958de 100644
--- a/test/unittest/allocatorstest.cpp
+++ b/test/unittest/allocatorstest.cpp
@@ -81,3 +81,22 @@ TEST(Allocator, Alignment) {
     }
 #endif
 }
+
+TEST(Allocator, Issue399) {
+    MemoryPoolAllocator<> a;
+    void* p = a.Malloc(100);
+    void* q = a.Realloc(p, 100, 200);
+    EXPECT_EQ(p, q);
+
+    // exhuasive testing
+    for (size_t j = 1; j < 32; j++) {
+        a.Clear();
+        a.Malloc(j); // some unaligned size
+        p = a.Malloc(1);
+        for (size_t i = 1; i < 1024; i++) {
+            q = a.Realloc(p, i, i + 1);
+            EXPECT_EQ(p, q);
+            p = q;
+        }
+    }
+}