diff --git a/pkgs/racket-test/tests/json/json.rkt b/pkgs/racket-test/tests/json/json.rkt
index 940567756c..e1652627e9 100644
--- a/pkgs/racket-test/tests/json/json.rkt
+++ b/pkgs/racket-test/tests/json/json.rkt
@@ -199,6 +199,12 @@
         (string->jsexpr @T{ falsetto }) =error> "string->jsexpr:"
         (string->jsexpr @T{ nullity }) =error> "string->jsexpr:"
         (string->jsexpr @T{ nulliparous }) =error> "string->jsexpr:"
+        (let ()
+          (define-values (in out) (make-pipe))
+          (display "started" out)
+          (flush-output out)
+          (read-json in))
+        =error> #rx"read-json: bad input starting #\"started\""
         ))
 
 (test do (pred-tests)
diff --git a/racket/collects/json/main.rkt b/racket/collects/json/main.rkt
index da7ae1abd7..a348e0aadb 100644
--- a/racket/collects/json/main.rkt
+++ b/racket/collects/json/main.rkt
@@ -466,16 +466,17 @@
       [else (bad-input)]))
   ;;
   (define (bad-input [prefix #""] #:eof? [eof? #f])
-    (define bstr (peek-bytes (sub1 (error-print-width)) 0 i))
-    (if (or (and (eof-object? bstr) (equal? prefix #""))
+    (define bstr (make-bytes (sub1 (error-print-width))))
+    (define bytes-read (peek-bytes-avail! bstr 0 #f i))
+    (if (or (and (eof-object? bytes-read) (equal? prefix #""))
             eof?)
         (err (string-append "unexpected end-of-file"
                             (if (equal? prefix #"")
                                 ""
                                 (format "after ~e" prefix))))
-        (err (format "bad input starting ~e" (bytes-append prefix (if (eof-object? bstr)
-                                                                      #""
-                                                                      bstr))))))
+        (err (format "bad input starting ~e" (bytes-append prefix (if (number? bytes-read)
+                                                                      (subbytes bstr 0 bytes-read)
+                                                                      #""))))))
   ;;
   (read-json #t))