diff --git a/include/chibi/sexp.h b/include/chibi/sexp.h
index 3f30cfa2..cd07f3fc 100755
--- a/include/chibi/sexp.h
+++ b/include/chibi/sexp.h
@@ -635,6 +635,12 @@ sexp sexp_make_flonum(sexp ctx, double f);
 
 #define sexp_portp(x) (sexp_iportp(x) || sexp_oportp(x))
 
+#if SEXP_USE_STRING_STREAMS
+#define sexp_stream_portp(x) 1
+#else
+#define sexp_stream_portp(x) (sexp_port_stream(x) != NULL)
+#endif
+
 /***************************** constructors ****************************/
 
 #define sexp_make_boolean(x) ((x) ? SEXP_TRUE : SEXP_FALSE)
diff --git a/lib/chibi/io/port.c b/lib/chibi/io/port.c
index 1d5b33b6..c294d4b1 100644
--- a/lib/chibi/io/port.c
+++ b/lib/chibi/io/port.c
@@ -115,7 +115,7 @@ static int sexp_cookie_seeker (void *cookie, off64_t *position, int whence) {
   sexp_gc_release2(ctx);
   return sexp_fixnump(res);
 }
-#endif
+#endif  /* !SEXP_BSD */
 
 static int sexp_cookie_cleaner (void *cookie) {
   sexp vec = (sexp)cookie, ctx, res;
@@ -141,7 +141,7 @@ static cookie_io_functions_t sexp_cookie_no_seek = {
   .close = (cookie_close_function_t*)sexp_cookie_cleaner,
 };
 
-#endif
+#endif  /* !SEXP_BSD */
 
 static sexp sexp_make_custom_port (sexp ctx, sexp self, char *mode,
                                    sexp read, sexp write,
@@ -184,7 +184,7 @@ static sexp sexp_make_custom_port (sexp ctx, sexp self, char *mode,
   return res;
 }
 
-#else
+#else  /* ! SEXP_USE_STRING_STREAMS */
 
 static sexp sexp_make_custom_port (sexp ctx, sexp self,
                                    char *mode, sexp read, sexp write,
@@ -202,7 +202,8 @@ static sexp sexp_make_custom_input_port (sexp ctx, sexp self,
 static sexp sexp_make_custom_output_port (sexp ctx, sexp self,
                                           sexp write, sexp seek, sexp close) {
   sexp res = sexp_make_custom_port(ctx, self, "w", SEXP_FALSE, write, seek, close);
-  sexp_pointer_tag(res) = SEXP_OPORT;
+  if (!sexp_exceptionp(res))
+    sexp_pointer_tag(res) = SEXP_OPORT;
   return res;
 }
 
diff --git a/tools/chibi-ffi b/tools/chibi-ffi
index 5ffaaab1..06102c8d 100755
--- a/tools/chibi-ffi
+++ b/tools/chibi-ffi
@@ -762,6 +762,13 @@
    args))
 
 (define (write-additional-checks args)
+  (for-each
+   (lambda (a)
+     (if (port-type? (type-base a))
+         (cat "  if (!sexp_stream_portp(arg" (type-index a) "))\n"
+              "    return sexp_xtype_exception(ctx, self,"
+              " \"not a FILE* backed port\", arg" (type-index a) ");\n")))
+   args)
   (for-each
    (lambda (a)
      (if (eq? 'input-port (type-base a))