diff --git a/ergotree-interpreter/src/eval/func_value.rs b/ergotree-interpreter/src/eval/func_value.rs
index f539db222..8fadd88b6 100644
--- a/ergotree-interpreter/src/eval/func_value.rs
+++ b/ergotree-interpreter/src/eval/func_value.rs
@@ -9,9 +9,78 @@ use crate::eval::Evaluable;
 
 impl Evaluable for FuncValue {
     fn eval<'ctx>(&self, _env: &mut Env, _ctx: &Context<'ctx>) -> Result<Value<'ctx>, EvalError> {
+        // The JVM supports only unary functions: FuncValue.eval
+        // (values.scala:1042-1056) builds the closure iff args.length == 1 and
+        // otherwise throws "Function must have 1 argument". The reject fires at
+        // closure CREATION (BlockValue evaluates ValDefs eagerly), so a bound
+        // non-unary lambda rejects even when never applied — but NOT at parse:
+        // both serializers are arity-agnostic, and a non-unary lambda on the
+        // dead branch of a lazy `if` must still accept.
+        if self.args().len() != 1 {
+            return Err(EvalError::Misc(format!(
+                "Function must have 1 argument, but was: {self:?}"
+            )));
+        }
         Ok(Value::Lambda(Lambda {
             args: self.args().to_vec(),
             body: self.body().clone().into(),
         }))
     }
 }
+
+#[cfg(test)]
+#[allow(clippy::unwrap_used)]
+mod tests {
+    use crate::eval::test_util::try_eval_out_with_version;
+    use ergotree_ir::chain::context::Context;
+    use ergotree_ir::ergo_tree::ErgoTree;
+    use ergotree_ir::serialization::SigmaSerializable;
+    use sigma_test_util::force_any_val;
+
+    fn hx(s: &str) -> alloc::vec::Vec<u8> {
+        (0..s.len())
+            .step_by(2)
+            .map(|i| u8::from_str_radix(&s[i..i + 2], 16).unwrap())
+            .collect()
+    }
+
+    // SANTA FuncValue.non_unary_arity (eval/v5/authored, JVM-blessed): the
+    // closure-creation arity gate, end-to-end from the vector bytes (v0 trees,
+    // no size bit — the unsized parse path takes the Int roots as-is).
+    #[test]
+    fn non_unary_lambda_rejects_at_closure_creation() {
+        let ctx = force_any_val::<Context>();
+        let run = |hex: &str| -> Result<i32, alloc::string::String> {
+            let tree = ErgoTree::sigma_parse_bytes(&hx(hex))
+                .map_err(|e| alloc::format!("parse: {e:?}"))?;
+            let expr = tree
+                .proposition()
+                .map_err(|e| alloc::format!("proposition: {e:?}"))?;
+            try_eval_out_with_version::<i32>(&expr, &ctx, 0, 2)
+                .map_err(|e| alloc::format!("eval: {e:?}"))
+        };
+        // { val add = (x:Int,y:Int) => x+y; add(3,4) }  — applied
+        // { val add = (x:Int,y:Int) => x+y; 5 }         — bound, never applied
+        //                                                  (ValDefs eval eagerly)
+        // { val f = () => 5; f() }                      — the zero-arg side
+        for hex in [
+            "100204060408d801d601d902020403049a72027203da72010273007301",
+            "1001040ad801d601d902020403049a720272037300",
+            "1001040ad801d601d9007300da720100",
+        ] {
+            assert!(
+                run(hex).is_err(),
+                "{hex}: non-unary lambda must reject at closure creation"
+            );
+        }
+        // if (true) 5 else { val add = (x:Int,y:Int) => x+y; add(3,4) } — the
+        // 2-arg lambda sits on the dead branch of a lazy If and never evaluates:
+        // the tree ACCEPTS (the gate is eval-time, not parse-time).
+        assert_eq!(
+            run("10040101040a040604089573007301d801d601d902020403049a72027203da72010273027303")
+                .unwrap(),
+            5,
+            "lazy-if twin: a never-evaluated non-unary lambda must not reject the tree"
+        );
+    }
+}