--- a/inst/@quaternion/subsasgn.m
+++ b/inst/@quaternion/subsasgn.m
@@ -38,7 +38,7 @@
 
     case "."                                                     # q.w = val
       if (! is_real_array (val))
-        error ("quaternion: subsasgn: invalid argument type, require real array");
+        error ("quaternion: subsasgn: invalid argument type, require real-valued array");
       endif
       if (! size_equal (subsref (q.w, idx(2:end)), val))
         error ("quaternion: subsasgn: invalid argument size [%s], require dimensions [%s]", ...