diff --git a/pkgs/racket-test-core/tests/racket/numstrs.rktl b/pkgs/racket-test-core/tests/racket/numstrs.rktl
index bf01770a18..e6d7afb2dd 100644
--- a/pkgs/racket-test-core/tests/racket/numstrs.rktl
+++ b/pkgs/racket-test-core/tests/racket/numstrs.rktl
@@ -53,6 +53,7 @@
     (-0.0 "-1#e-10000000000000000000000000000000")
     (1e-134
      "0.00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001")
+    (-0.0 "-632.3206524753840966914228247597079196012717570277972845484e-399")
     (10.0 "1#")
     (10.0 "1#e0")
     (10.0 "1####e-3")
diff --git a/racket/src/racket/src/numstr.c b/racket/src/racket/src/numstr.c
index a3817d101b..ac7b837b45 100644
--- a/racket/src/racket/src/numstr.c
+++ b/racket/src/racket/src/numstr.c
@@ -1481,7 +1481,8 @@ Scheme_Object *scheme_read_number(const mzchar *str, intptr_t len,
 
         max_useful = scheme_bin_plus(scheme_make_integer(MAX_FLOATREAD_PRECISION_DIGITS(is_long_double)),
                                      exponent);
-        if (scheme_bin_lt(max_useful, scheme_make_integer(0))) {
+        if (scheme_bin_lt(max_useful, scheme_make_integer(2))) {
+          /* will definitely underflow */
           if (dcp > 2)
             dcp = 2; /* leave room for a sign and a digit */
         } else if (SCHEME_INTP(max_useful)) {