diff --git a/src/main/java/com/amazon/ion/impl/IonReaderTextRawTokensX.java b/src/main/java/com/amazon/ion/impl/IonReaderTextRawTokensX.java
index 9a7a445a6..18743648b 100644
--- a/src/main/java/com/amazon/ion/impl/IonReaderTextRawTokensX.java
+++ b/src/main/java/com/amazon/ion/impl/IonReaderTextRawTokensX.java
@@ -2582,6 +2582,13 @@ private final int read_escaped_char_content_helper(int c1, boolean is_clob)
                 bad_escape_sequence(c2);
             }
             c2 = read_hex_escape_sequence_value(8);
+            // Eight hex digits can set the sign bit, wrapping the assembled
+            // value negative. A negative result is always out of the Unicode
+            // range and would otherwise slip past the downstream
+            // Character.MAX_CODE_POINT check, decoding to an unrelated char.
+            if (c2 < 0) {
+                error("invalid code point in \\U escape sequence");
+            }
             break;
         case IonTokenConstsX.ESCAPE_HEX:
             c2 = read_hex_escape_sequence_value(2);
diff --git a/src/test/java/com/amazon/ion/StringTest.java b/src/test/java/com/amazon/ion/StringTest.java
index 79dff34f1..782f275ed 100644
--- a/src/test/java/com/amazon/ion/StringTest.java
+++ b/src/test/java/com/amazon/ion/StringTest.java
@@ -1,18 +1,5 @@
-/*
- * Copyright 2007-2019 Amazon.com, Inc. or its affiliates. All Rights Reserved.
- *
- * Licensed under the Apache License, Version 2.0 (the "License").
- * You may not use this file except in compliance with the License.
- * A copy of the License is located at
- *
- *     http://www.apache.org/licenses/LICENSE-2.0
- *
- * or in the "license" file accompanying this file. This file is distributed
- * on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either
- * express or implied. See the License for the specific language governing
- * permissions and limitations under the License.
- */
-
+// Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+// SPDX-License-Identifier: Apache-2.0
 package com.amazon.ion;
 
 import org.junit.Test;
@@ -215,6 +202,22 @@ public void testTrickyEscapes()
         checkString(expected, value);
     }
 
+    @Test
+    public void testBigUEscapeOutOfRange()
+    {
+        // Eight hex digits with the high bit set wrap the assembled value
+        // negative, which must still be rejected rather than decoded to a
+        // stray character (previously \\U80000000..\\UFFFFFFFF yielded U+FFFF).
+        badValue("\"\\U80000000\"");
+        badValue("\"\\UFFFFFFFF\"");
+        // Positive values beyond the Unicode range stay rejected.
+        badValue("\"\\U00110000\"");
+        // The maximum valid scalar is still accepted.
+        IonString value = (IonString) oneValue("\"\\U0010FFFF\"");
+        assertEquals(new StringBuilder().appendCodePoint(0x10FFFF).toString(),
+                     value.stringValue());
+    }
+
     @Test
     public void testReadStringsFromSuite()
         throws Exception