diff --git a/src/kernel/unit.c b/src/kernel/unit.c
index d922efcd5..ba2543e2b 100644
--- a/src/kernel/unit.c
+++ b/src/kernel/unit.c
@@ -735,6 +735,7 @@ variant read_unit_reference(struct storage * store)
 
 int get_level(const unit * u, skill_t id)
 {
+    assert(id != NOSKILL);
     if (skill_enabled(id)) {
         skill *sv = u->skills;
         while (sv != u->skills + u->skill_size) {
diff --git a/src/skill.c b/src/skill.c
index 3bee18005..3d5b77a96 100644
--- a/src/skill.c
+++ b/src/skill.c
@@ -45,6 +45,7 @@ const char *skillnames[MAXSKILLS] = {
 bool skill_disabled[MAXSKILLS];
 
 bool skill_enabled(skill_t sk) {
+    assert(sk != NOSKILL);
     return !skill_disabled[sk];
 }