diff --git a/lib_debug/debug.c b/lib_debug/debug.c
index 736f35d43d3b41ca29428063ff654f396e18e3e1..c6a5f8c5efa24502f2b76d2e4d33c4b937ec49eb 100644
--- a/lib_debug/debug.c
+++ b/lib_debug/debug.c
@@ -39,6 +39,7 @@
 #include "options.h"
 #ifdef DEBUGGING
 #include "debug.h"
+#include <assert.h>
 #ifdef DEBUG_MODE_INFO
 #ifdef DEBUG_MODE_INFO_TWEAK
 #include <sys/stat.h>
@@ -771,8 +772,10 @@ char *fname(
     const int16_t id,
     const int16_t enc_dec )
 {
-    char idd[5] = ".idX";
-    idd[3] = (char) ( id + '0' );
+    char idd[6];
+
+    assert( id < 100 );
+    sprintf( idd, ".id%d", id );
 
     strcpy( tmp_fname, dir );
     strcat( tmp_fname, file );