diff --git a/appl/popper/popper.h b/appl/popper/popper.h
index 7da014910..6fe7ffb41 100644
--- a/appl/popper/popper.h
+++ b/appl/popper/popper.h
@@ -342,7 +342,7 @@ __attribute__ ((format (printf, 3, 4)))
 #endif
 ;
 
-int pop_msg(POP *p, int stat, char *format, ...)
+int pop_msg(POP *p, int stat, const char *format, ...)
 #ifdef __GNUC__
 __attribute__ ((format (printf, 3, 4)))
 #endif