diff --git a/scsh/fdports1.c b/scsh/fdports1.c
index 78562ac..6dc00e5 100644
--- a/scsh/fdports1.c
+++ b/scsh/fdports1.c
@@ -394,8 +394,9 @@ int read_fdport_substring(scheme_value buf, int start, int end, scheme_value dat
 int write_fdport_substring(scheme_value buf, int start, int end, scheme_value data)
 {
     FILE *f = fstar_cache[EXTRACT_FIXNUM(*PortData_Fd(data))];
-    int retval = fwrite(StrByte(buf,start), 1, end-start, f);
-    return ferror(f) ? -1 : retval; /* -1: error, 0: eof */
+    int nbytes = end - start;
+    int retval = fwrite(StrByte(buf,start), 1, nbytes, f);
+    return (retval < nbytes) ? -1 : retval; /* -1: error, otw numwritten */
     }
 
 /* 1st return value says why we terminated the read: