diff --git a/source/common/unistr.cpp b/source/common/unistr.cpp
index 04f01cfa..626f8886 100644
--- a/source/common/unistr.cpp
+++ b/source/common/unistr.cpp
@@ -1865,6 +1865,13 @@ UnicodeString::cloneArrayIfNeeded(int32_t newCapacity,
       growCapacity = newCapacity;
     } else if(newCapacity <= US_STACKBUF_SIZE && growCapacity > US_STACKBUF_SIZE) {
       growCapacity = US_STACKBUF_SIZE;
+    } else if(newCapacity > growCapacity) {
+      setToBogus();
+      return false;  // bad inputs
+    }
+    if(growCapacity > kMaxCapacity) {
+      setToBogus();
+      return false;
     }
 
     // save old values
diff --git a/source/test/intltest/ustrtest.cpp b/source/test/intltest/ustrtest.cpp
index 26c51b9c..71ebde4b 100644
--- a/source/test/intltest/ustrtest.cpp
+++ b/source/test/intltest/ustrtest.cpp
@@ -68,6 +68,7 @@ void UnicodeStringTest::runIndexedTest( int32_t index, UBool exec, const char* &
     TESTCASE_AUTO(TestNullPointers);
     TESTCASE_AUTO(TestUnicodeStringInsertAppendToSelf);
     TESTCASE_AUTO(TestLargeAppend);
+    TESTCASE_AUTO(TestLargeMemory);
     TESTCASE_AUTO_END;
 }
 
@@ -2328,6 +2329,18 @@ void UnicodeStringTest::TestUnicodeStringInsertAppendToSelf() {
     assertEquals("", u"abbcdcde", str);
 }
 
+void UnicodeStringTest::TestLargeMemory() {
+#if U_PLATFORM_IS_LINUX_BASED || U_PLATFORM_IS_DARWIN_BASED
+    if(quick) { return; }
+    IcuTestErrorCode status(*this, "TestLargeMemory");
+    constexpr uint32_t len = 2147483643;
+    char16_t *buf = new char16_t[len];
+    if (buf == nullptr) { return; }
+    uprv_memset(buf, 0x4e, len * 2);
+    icu::UnicodeString test(buf, len);
+    delete [] buf;
+#endif
+}
 void UnicodeStringTest::TestLargeAppend() {
     if(quick) return;
 
@@ -2356,15 +2369,17 @@ void UnicodeStringTest::TestLargeAppend() {
     }
     dest.remove();
     total = 0;
+    // Copy kMaxCapacity from common/unistr.cpp
+    const int32_t kMaxCapacity = 0x7ffffff5;
     for (int32_t i = 0; i < 16; i++) {
         dest.append(str);
         total += len;
-        if (total + len <= INT32_MAX) {
+        if (total + len <= kMaxCapacity) {
             assertFalse("dest is not bogus", dest.isBogus());
-        } else if (total <= INT32_MAX) {
+        } else if (total <= kMaxCapacity) {
             // Check that a string of exactly the maximum size works
             UnicodeString str2;
-            int32_t remain = static_cast<int32_t>(INT32_MAX - total);
+            int32_t remain = static_cast<int32_t>(kMaxCapacity - total);
             char16_t *buf2 = str2.getBuffer(remain);
             if (buf2 == nullptr) {
                 // if somehow memory allocation fail, return the test
@@ -2374,14 +2389,19 @@ void UnicodeStringTest::TestLargeAppend() {
             str2.releaseBuffer(remain);
             dest.append(str2);
             total += remain;
-            assertEquals("When a string of exactly the maximum size works", (int64_t)INT32_MAX, total);
-            assertEquals("When a string of exactly the maximum size works", INT32_MAX, dest.length());
+            assertEquals("When a string of exactly the maximum size works", static_cast<int64_t>(kMaxCapacity), total);
+            assertEquals("When a string of exactly the maximum size works", kMaxCapacity, dest.length());
             assertFalse("dest is not bogus", dest.isBogus());
 
-            // Check that a string size+1 goes bogus
+            // Check that a string size+1 does not go bogus (one more byte reserved for NUL)
             str2.truncate(1);
             dest.append(str2);
             total++;
+            assertFalse("dest should be not bogus", dest.isBogus());
+            // Check that a string size+2 goes bogus (beyond the byte reserved
+            // for NUL)
+            dest.append(str2);
+            total++;
             assertTrue("dest should be bogus", dest.isBogus());
         } else {
             assertTrue("dest should be bogus", dest.isBogus());
diff --git a/source/test/intltest/ustrtest.h b/source/test/intltest/ustrtest.h
index 088b7139..9e50d365 100644
--- a/source/test/intltest/ustrtest.h
+++ b/source/test/intltest/ustrtest.h
@@ -92,6 +92,7 @@ public:
     void TestUnicodeStringImplementsAppendable();
     void TestSizeofUnicodeString();
     void TestMoveSwap();
+    void TestLargeMemory();
 
     void TestUInt16Pointers();
     void TestWCharPointers();