diff --git a/skeletons/BOOLEAN.c b/skeletons/BOOLEAN.c
index 3da5a6b..fd8b80c 100644
--- a/skeletons/BOOLEAN.c
+++ b/skeletons/BOOLEAN.c
@@ -406,7 +406,8 @@
         }
     }
 
-    if(!constraints) constraints = &td->encoding_constraints;
+    if(!constraints || !constraints->per_constraints)
+        constraints = &td->encoding_constraints;
     if(constraints->per_constraints) {
         const asn_per_constraint_t *pc = &constraints->per_constraints->value;
         if(pc->flags & APC_CONSTRAINED) {