diff --git a/src/flow/binary.rs b/src/flow/binary.rs
index 06693ab..1d2b4e3 100644
--- a/src/flow/binary.rs
+++ b/src/flow/binary.rs
@@ -3,6 +3,8 @@ pub mod balancing;
 pub mod bound;
 pub mod bounds;
 
+use std::fmt::Display;
+
 use crate::flow::comparator::*;
 use crate::func::{context::*, *};
 
@@ -132,10 +134,26 @@ pub trait BinaryTreesMutable<'a>: BinaryTreesEmpty<'a> + BinaryTreesTreeOf<'a> {
     }
 }
 
+#[derive(Debug)]
+pub enum HeightError {
+    LeafHeight(u64),
+    NodeHeight,
+}
+
+impl Display for HeightError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Self::NodeHeight => write!(f, "invalid node height: 0."),
+            Self::LeafHeight(height) => {
+                write!(f, "invalid leaf height: {height}!=0.")
+            }
+        }
+    }
+}
+
 pub trait BinaryTreesHeight<'a>: BinaryTrees<'a> {
     fn height(&self, tree: &Self::Tree) -> u64;
-    fn leaf_height_error<T: 'a>(&self, height: u64) -> BTWrap<'a, Self, T>;
-    fn node_height_error<T: 'a>(&self) -> BTWrap<'a, Self, T>;
+    fn height_error<T: 'a>(&self, error: HeightError) -> BTWrap<'a, Self, T>;
 }
 
 pub trait BinaryTreesTryJoin<'a>: BinaryTrees<'a> {
diff --git a/src/flow/binary/avl.rs b/src/flow/binary/avl.rs
index ede31bf..9acd5d7 100644
--- a/src/flow/binary/avl.rs
+++ b/src/flow/binary/avl.rs
@@ -6,7 +6,7 @@ pub trait BinaryTreesAvl<'a>:
     fn assume_node(&self, tree: &Self::Tree) -> BTWrap<'a, Self, Self::Node> {
         match self.refer(tree) {
             Some(reference) => self.resolve(&reference),
-            None => self.leaf_height_error(self.height(tree)),
+            None => self.height_error(HeightError::LeafHeight(self.height(tree))),
         }
     }
 
diff --git a/src/flow/binary/balancing.rs b/src/flow/binary/balancing.rs
index a8b95b7..26b898a 100644
--- a/src/flow/binary/balancing.rs
+++ b/src/flow/binary/balancing.rs
@@ -160,12 +160,8 @@ impl<'a, BT: BinaryTreesUnbalanced<'a>> BinaryTreesHeight<'a> for BalancedTrees<
         self.0.height(tree)
     }
 
-    fn leaf_height_error<T: 'a>(&self, height: u64) -> BTWrap<'a, Self, T> {
-        self.0.leaf_height_error(height)
-    }
-
-    fn node_height_error<T: 'a>(&self) -> BTWrap<'a, Self, T> {
-        self.0.node_height_error()
+    fn height_error<T: 'a>(&self, error: HeightError) -> BTWrap<'a, Self, T> {
+        self.0.height_error(error)
     }
 }
 
diff --git a/src/flow/binary/bound.rs b/src/flow/binary/bound.rs
index 72b7bf9..0f655ae 100644
--- a/src/flow/binary/bound.rs
+++ b/src/flow/binary/bound.rs
@@ -169,12 +169,8 @@ impl<'a, BT: BinaryTreesBindable<'a> + BinaryTreesHeight<'a>> BinaryTreesHeight<
         self.0.height(&tree.bound)
     }
 
-    fn leaf_height_error<T: 'a>(&self, height: u64) -> BTWrap<'a, Self, T> {
-        self.0.leaf_height_error(height)
-    }
-
-    fn node_height_error<T: 'a>(&self) -> BTWrap<'a, Self, T> {
-        self.0.node_height_error()
+    fn height_error<T: 'a>(&self, error: HeightError) -> BTWrap<'a, Self, T> {
+        self.0.height_error(error)
     }
 }
 
diff --git a/src/mrds/trees/heighted.rs b/src/mrds/trees/heighted.rs
index 5822f2c..159a773 100644
--- a/src/mrds/trees/heighted.rs
+++ b/src/mrds/trees/heighted.rs
@@ -119,12 +119,8 @@ impl<'a, A: 'a + Ord + Clone> BinaryTreesHeight<'a> for Trees<A> {
         tree.height
     }
 
-    fn leaf_height_error<T: 'a>(&self, height: u64) -> BTWrap<'a, Self, T> {
-        panic!("leaf height error: {height}.")
-    }
-
-    fn node_height_error<T: 'a>(&self) -> BTWrap<'a, Self, T> {
-        panic!("node with height 0.")
+    fn height_error<T: 'a>(&self, error: HeightError) -> BTWrap<'a, Self, T> {
+        panic!("{error}")
     }
 }