diff --git a/crates/web/src/lib.rs b/crates/web/src/lib.rs
index 4f09527..68ac572 100644
--- a/crates/web/src/lib.rs
+++ b/crates/web/src/lib.rs
@@ -945,7 +945,7 @@ impl FuncBuilder {
     /// integer type; or if `t` is an integer type which cannot represent the given value of `x`.
     pub fn num(&mut self, t: usize, x: f64) -> Result<usize, JsError> {
         match self.ty(t)? {
-            Ty::F64 => Ok(self.constant(t, rose::Expr::F64 { val: x })),
+            Ty::F64 | Ty::T64 => Ok(self.constant(t, rose::Expr::F64 { val: x })),
             &Ty::Fin { size } => {
                 let y = x as usize;
                 if y as f64 != x {
diff --git a/packages/core/src/index.test.ts b/packages/core/src/index.test.ts
index 23c2df5..f44d665 100644
--- a/packages/core/src/index.test.ts
+++ b/packages/core/src/index.test.ts
@@ -433,6 +433,13 @@ describe("valid", () => {
     expect(interp(g)(0, 1)).toBeCloseTo(50000);
   });
 
+  test("custom JVP with zero tangent", () => {
+    const signum = opaque([Real], Real, Math.sign);
+    signum.jvp = fn([Dual], Dual, ({ re: x }) => ({ re: sign(x), du: 0 }));
+    const f = interp(jvp(fn([Real], Real, (x) => signum(x))));
+    expect(f({ re: 2, du: 1 })).toEqual({ re: 1, du: 0 });
+  });
+
   test("VJP", () => {
     const f = fn([Vec(2, Real)], Real, (v) => mul(v[0], v[1]));
     const g = fn([], Vec(3, Real), () => {