diff --git a/mats/fl.ms b/mats/fl.ms
index 2dd2a3760f..cd9a8a635f 100644
--- a/mats/fl.ms
+++ b/mats/fl.ms
@@ -26,6 +26,13 @@
         (flonum->fixnum (* (+ (ash (most-positive-fixnum) -1) 1) 1.0)))
    (eq? (most-negative-fixnum)
         (flonum->fixnum (* (most-negative-fixnum) 1.0)))
+   (eq? (+ (ash (most-positive-fixnum) -1) 1)
+        (flonum->fixnum (fl+ (* (+ (ash (most-positive-fixnum) -1) 1) 1.0) 0.5)))
+   (or (not (fixnum? (inexact->exact (exact->inexact (most-positive-fixnum)))))
+       (eq? (most-positive-fixnum)
+            (flonum->fixnum (fl+ (* (most-positive-fixnum) 1.0) 0.5))))
+   (eq? (most-negative-fixnum)
+        (flonum->fixnum (fl- (* (most-negative-fixnum) 1.0) 0.5)))
    (eq? (flonum->fixnum 0.0) 0)
    (eq? (flonum->fixnum 1.0) 1)
    (eq? (flonum->fixnum +4.5) +4)
diff --git a/s/cpnanopass.ss b/s/cpnanopass.ss
index 3205aab5fa..5788caca01 100644
--- a/s/cpnanopass.ss
+++ b/s/cpnanopass.ss
@@ -7411,7 +7411,7 @@
                            (%inline sll ,body (immediate ,(fx- 0 cnt)))
                            body)))
                   (immediate ,mask)))))
-<
+
           (define-inline 3 fllp
             [(e) (build-flonum-extractor 19 12 e)])
 
diff --git a/s/mathprims.ss b/s/mathprims.ss
index 741eea4edc..8401e2f3ae 100644
--- a/s/mathprims.ss
+++ b/s/mathprims.ss
@@ -317,11 +317,25 @@
          ($flonum-sign x)))
 
    (set-who! flonum->fixnum
-      (let ([flmnf (fixnum->flonum (most-negative-fixnum))]
-            [flmpf (fixnum->flonum (most-positive-fixnum))])
+      (let ([flmnf (meta-cond
+                    ;; 64-bit fixnums: -1.0 is the same flonum
+                    [(fl= (fixnum->flonum (most-negative-fixnum))
+                          (fl- (fixnum->flonum (most-negative-fixnum)) 1.0))
+                     ;; Find the next lower flonum:
+                     (let loop ([amt 2.0])
+                       (let ([v (fl- (fixnum->flonum (most-negative-fixnum)) amt)])
+                         (if (fl= v (fixnum->flonum (most-negative-fixnum)))
+                             (loop (fl* 2.0 amt))
+                             v)))]
+                    [else
+                     (fl- (fixnum->flonum (most-negative-fixnum)) 1.0)])]
+            ;; Although adding 1.0 doesn't change the flonum for
+            ;; 64-bit fixnums, the flonum doesn't fit in a fixnum, so
+            ;; this is the upper bbound we want either way:
+            [flmpf (fl+ (fixnum->flonum (most-positive-fixnum)) 1.0)])
          (lambda (x)
             (unless (flonum? x) (flargerr who x))
-            (unless (fl<= flmnf x flmpf)
+            (unless (fl< flmnf x flmpf)
                ($oops who "result for ~s would be outside of fixnum range" x))
             (#3%flonum->fixnum x))))
 )