diff --git a/lib_debug/debug.c b/lib_debug/debug.c
index 440d1f7a06589e6951fb78c88d0faec227338328..d8cdd6c8b1b770394dd9d421f7f4367257149f43 100644
--- a/lib_debug/debug.c
+++ b/lib_debug/debug.c
@@ -779,11 +779,13 @@ char *fname(
     const int16_t id,
     const int16_t enc_dec )
 {
-    char idd[5] = ".idX";
-    idd[3] = (char) ( id + '0' );
 #ifdef DEBUG_FORCE_DIR
     size_t len;
 #endif
+    char idd[6];
+
+    assert( id < 100 );
+    sprintf( idd, ".id%d", id );
 
     strcpy( tmp_fname, dir );
 #ifdef DEBUG_FORCE_DIR