diff --git a/proton-j/src/main/java/org/apache/qpid/proton/codec/StringType.java b/proton-j/src/main/java/org/apache/qpid/proton/codec/StringType.java
index 092894d..a035e94 100644
--- a/proton-j/src/main/java/org/apache/qpid/proton/codec/StringType.java
+++ b/proton-j/src/main/java/org/apache/qpid/proton/codec/StringType.java
@@ -97,7 +97,7 @@
                 {
                     len++;
                     // surrogate pairs should always combine to create a code point with a 4 octet representation
-                    if ((c & 0xD800) == 0xD800)
+                    if ((c & 0xD800) == 0xD800 && c < 0xDC00)
                     {
                         i++;
                     }
diff --git a/proton-j/src/test/java/org/apache/qpid/proton/codec/StringTypeTest.java b/proton-j/src/test/java/org/apache/qpid/proton/codec/StringTypeTest.java
index 7d78f65..7a44063 100644
--- a/proton-j/src/test/java/org/apache/qpid/proton/codec/StringTypeTest.java
+++ b/proton-j/src/test/java/org/apache/qpid/proton/codec/StringTypeTest.java
@@ -140,9 +140,17 @@
                                                          UnicodeBlock.MUSICAL_SYMBOLS,
                                                          /*UnicodeBlock.EMOTICONS,*/
                                                          /*UnicodeBlock.PLAYING_CARDS,*/
+                                                         UnicodeBlock.BOX_DRAWING,
                                                          UnicodeBlock.HALFWIDTH_AND_FULLWIDTH_FORMS,
                                                          UnicodeBlock.SUPPLEMENTARY_PRIVATE_USE_AREA_A,
                                                          UnicodeBlock.SUPPLEMENTARY_PRIVATE_USE_AREA_B));
+                    // some additional combinations of characters that could cause problems to the encoder
+                    String[] boxDrawing = getAllStringsFromUnicodeBlocks(UnicodeBlock.BOX_DRAWING).toArray(new String[0]);
+                    String[] halfFullWidthForms = getAllStringsFromUnicodeBlocks(UnicodeBlock.HALFWIDTH_AND_FULLWIDTH_FORMS).toArray(new String[0]);
+                    for (int i = 0; i < halfFullWidthForms.length; i++)
+                    {
+                        add(halfFullWidthForms[i] + boxDrawing[i % boxDrawing.length]);
+                    }
                 }
             };
     }