diff --git a/json/src/JSON/Internal.ss b/json/src/JSON/Internal.ss
index 8cfb4060..a2fe92c6 100644
--- a/json/src/JSON/Internal.ss
+++ b/json/src/JSON/Internal.ss
@@ -194,7 +194,7 @@
                   (fxsll (fx- w1 #xD800) 10)
                   (fx- w2 #xDC00))
                 #x10000)
-              (error #f (format "Invalid unicode surrogate pair ~a ~a" w1 w2))))]
+              (error #f (format "Invalid unicode surrogate pair ~,x ~,x" w1 w2))))]
         [(fx<= #xDC00 w1 #xDFFF)
           (error #f (format "Invalid unicode escape ~,x" w1))]
         [else w1])))
diff --git a/json/test/Main.ss b/json/test/Main.ss
index 03d2d1c9..9cab4afa 100644
--- a/json/test/Main.ss
+++ b/json/test/Main.ss
@@ -69,6 +69,7 @@
       (assert-roundtrip "\" \\uD801\\uDC37 foo\"" "\" 𐐷 foo\"")
       (assert-fail "\"\\uD801 \"" "Unexpected ' ', was expecting '\\'")
       (assert-fail "\"\\uDC37 \"" "Invalid unicode escape DC37")
+      (assert-fail "\" \\uD801\\uD802 foo\"" "Invalid unicode surrogate pair D801 D802")
 
       (display "  Leading whitespace\n")
       (assert-roundtrip "\n\r  \"\\nbar\"" "\"\\nbar\"")