diff --git a/decode.lisp b/decode.lisp
index 17be1bd..7565478 100644
--- a/decode.lisp
+++ b/decode.lisp
@@ -363,7 +363,7 @@ the form which is used to obtain the next octet."
         (declare (inline read-next-word))
         (let ((word (read-next-word)))
           (declare (type (unsigned-byte 16) word))
-          (cond ((<= #xd800 word #xdfff)
+          (cond ((<= #xd800 word #xdbff)
                  (let ((next-word (read-next-word)))
                    (declare (type (unsigned-byte 16) next-word))
                    (unless (<= #xdc00 next-word #xdfff)
@@ -394,7 +394,7 @@ the form which is used to obtain the next octet."
         (declare (inline read-next-word))
         (let ((word (read-next-word)))
           (declare (type (unsigned-byte 16) word))
-          (cond ((<= #xd800 word #xdfff)
+          (cond ((<= #xd800 word #xdbff)
                  (let ((next-word (read-next-word)))
                    (declare (type (unsigned-byte 16) next-word))
                    (unless (<= #xdc00 next-word #xdfff)
diff --git a/test/test.lisp b/test/test.lisp
index 1d88641..06dca6f 100644
--- a/test/test.lisp
+++ b/test/test.lisp
@@ -558,7 +558,21 @@ the external format EXTERNAL-FORMAT."
         (format t "~&Illegal code points"))
       (want-encoding-error #(#x00 #x00 #x11 #x00) :utf-32le)
       (want-encoding-error #(#x00 #xd8) :utf-16le)
-      (want-encoding-error #(#xff #xdf) :utf-16le))
+      (want-encoding-error #(#xff #xdf) :utf-16le)
+      (when verbose
+        (format t "~&Low surrogates cannot be the first word of a surrogate pair"))
+      ;; The bug: decode.lisp incorrectly accepts low surrogates (#xdc00-#xdfff)
+      ;; as the first word. It should only accept high surrogates (#xd800-#xdbff).
+      ;; These tests verify that low surrogates followed by valid low surrogates
+      ;; are properly rejected (not incorrectly decoded as surrogate pairs).
+      (want-encoding-error #(#x00 #xdc #x00 #xdc) :utf-16le)
+      (want-encoding-error #(#x00 #xdc #xff #xdf) :utf-16le)
+      (want-encoding-error #(#xff #xdc #x00 #xdc) :utf-16le)
+      (want-encoding-error #(#x00 #xdd #x00 #xdd) :utf-16le)
+      (want-encoding-error #(#xff #xdf #xff #xdf) :utf-16le)
+      (want-encoding-error #(#xdc #x00 #xdc #x00) :utf-16be)
+      (want-encoding-error #(#xdf #xff #xdf #xff) :utf-16be)
+      (want-encoding-error #(#x00 #xdc #x41 #x00) :utf-16le))
     (macrolet ((want-encoding-error (input format)
                  `(with-expected-error (external-format-encoding-error)
                     (read-flexi-line* ,input ,format))))