--- a/units.c
+++ b/units.c
@@ -761,7 +761,7 @@
 }
 
 
-#define readerror (goterr=1) && errfile && fprintf
+#define readerror(...) do { goterr=1; if (errfile) fprintf(__VA_ARGS__); } while (0)
 
 #define VAGUE_ERR "%s: error in units file '%s' line %d\n", \
                        progname, file, linenum