diff --git a/solutions/fully-configurable/variables.tf b/solutions/fully-configurable/variables.tf
index 33c89dae..337ae968 100644
--- a/solutions/fully-configurable/variables.tf
+++ b/solutions/fully-configurable/variables.tf
@@ -47,7 +47,7 @@ variable "prefix" {
 
   validation {
     # must not exceed 16 characters in length
-    condition     = length(var.prefix) <= 16
+    condition     = var.prefix == null || var.prefix == "" ? true : length(var.prefix) <= 16
     error_message = "Prefix must not exceed 16 characters."
   }
 }