diff --git a/src/c/file.d b/src/c/file.d
index ce4167baa..21f8a925a 100755
--- a/src/c/file.d
+++ b/src/c/file.d
@@ -4812,7 +4812,8 @@ cl_file_string_length(cl_object stream, cl_object string)
     }
   }
   unlikely_if (!ECL_FILE_STREAM_P(stream)) {
-    not_a_file_stream(stream);
+    /* Spec says, that if cannot be determined we return NIL. */
+    return ECL_NIL;
   }
   switch (ecl_t_of(string)) {
 #ifdef ECL_UNICODE