diff --git a/code/reader.lisp b/code/reader.lisp
index 82ae7f07ea056620f115ea7984f079abf86d322f..7d6279cbbdccfd1f84c0c61f28e8571938ed6c70 100644
--- a/code/reader.lisp
+++ b/code/reader.lisp
@@ -5,7 +5,7 @@
 ;;; Carnegie Mellon University, and has been placed in the public domain.
 ;;;
 (ext:file-comment
-  "$Header: /Volumes/share2/src/cmucl/cvs2git/cvsroot/src/code/reader.lisp,v 1.56 2005/05/06 20:36:31 rtoy Exp $")
+  "$Header: /Volumes/share2/src/cmucl/cvs2git/cvsroot/src/code/reader.lisp,v 1.57 2005/10/21 13:11:59 rtoy Exp $")
 ;;;
 ;;; **********************************************************************
 ;;;
@@ -1538,8 +1538,17 @@ the end of the stream."
 	  (t (error "Internal error in floating point reader.")))))
 
 (defun make-float-aux (number divisor float-format stream)
-  (handler-case 
-      (coerce (/ number divisor) float-format)
+  (handler-case
+      (with-float-traps-masked (:underflow)
+	(let ((result (coerce (/ number divisor) float-format)))
+	  (when (and (zerop result) (not (zerop number)))
+	    ;; With underflow traps disabled, reading any number
+	    ;; smaller than least-positive-foo-float will return zero.
+	    ;; But we really want to indicate that we can't read it.
+	    ;; So if we converted the number to zero, but the number
+	    ;; wasn't actually zero, throw an error.
+	    (error "Underflow"))
+	  result))
     (error ()
 	   (%reader-error stream "Floating-point number not representable"))))