diff --git a/extflat/EFerr.c b/extflat/EFerr.c
index 280fb9ad..c69d3bf5 100644
--- a/extflat/EFerr.c
+++ b/extflat/EFerr.c
@@ -64,7 +64,7 @@ extern int Tcl_printf();
  */
 
 void
-efReadError(char *fmt, ...)
+efReadError(const char *fmt, ...)
 {
     va_list args;