1 files changed, 10 insertions, 5 deletions
diff --git a/src/r.bqn b/src/r.bqn
index a276ec68..d072d318 100644
--- a/src/r.bqn
+++ b/src/r.bqn
@@ -176,7 +176,7 @@ _under_←{
     Fail←{𝕊‿0}
     # 𝕎 is parent traversal; 𝕩 is current components of ind and val
     Trav←(IsArray 0⊑⊢)◶⟨Pair, Conform´∘⊢◶Fail‿{
-      Parent←𝕎 ⋄ n←≠⊑a←⥊¨𝕩 ⋄ j←¯1
+      Parent←𝕎 ⋄ n←≠0⊑a←⥊¨𝕩 ⋄ j←¯1
       Child←Trav⟜(⊑¨⟜a)
       { j+↩1 ⋄ f←n⊸≤◶⟨𝕊˙⊸Child,Parent˙⟩j ⋄ F 0 }
     }⟩
@@ -189,11 +189,16 @@ _under_←{
     Set1←𝕨⊸{
       𝕩↩<⍟(1-IsArray)𝕩
       l←1×´s←≢𝕩
-      gi←0⊸⊑⌜𝕨⋄nv←1⊸⊑⌜𝕨
-      g←Cmp0 _grade_ 0 gi
-      P←(≠g)⊸≤◶⟨(⊑⟜g)⊑gi˙,l⟩
+      i←0⊸⊑⌜𝕨
+      g←Cmp0 _grade_ 0 i
+      v←(1⊑⊑⟜𝕨)⌜g
+      P←(≠g)⊸≤◶⟨(⊑⟜g)⊑i˙,l⟩
       e←P j←0
-      s⥊{e=𝕩}◶⟨⊑⟜(⥊𝕩),{𝕩⋄r←(j⊑g)⊑nv⋄e↩P j↩1+j⋄r}⟩⌜↕l
+      s⥊{e=𝕩}◶⟨⊑⟜(⥊𝕩),{𝕩
+        r←j⊑v⋄e↩{𝕊∘{𝕩
+          "⌾: Incompatible result elements in structural Under"!r Match j⊑v
+        }⍟(e=⊢)P j↩1+j}0⋄r
+      }⟩⌜↕l
     }
     _at_ ← {(↕≠𝕩) 𝔽⍟((𝔾𝕩)=⊣)¨ 𝕩}
     Set ← 0⊸{ (𝕨≥≠root)◶⟨≢⥊(1+𝕨)⊸𝕊_at_(𝕨⊑root˙)∘⥊, Set1⟩ 𝕩 }