diff --git a/include/config_fallbacks.h b/include/config_fallbacks.h
index 07df99664c4..6a6350e0a14 100644
--- a/include/config_fallbacks.h
+++ b/include/config_fallbacks.h
@@ -82,7 +82,7 @@
 #endif
 
 #ifndef CONFIG_SYS_PBSIZE
-#define CONFIG_SYS_PBSIZE	(CONFIG_SYS_CBSIZE + 128)
+#define CONFIG_SYS_PBSIZE	(CONFIG_SYS_CBSIZE + sizeof(CONFIG_SYS_PROMPT) + 16)
 #endif
 
 #ifndef CONFIG_FIT_SIGNATURE