diff --git a/CHANGELOG.md b/CHANGELOG.md
index a8b3201034f..500c73e2d00 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -85,6 +85,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Fixed undefined behaviour caused by incorrect `ExactSizeIterator` implementations. [#2124](https://github.com/PyO3/pyo3/pull/2124)
 - Add missing FFI definitions `_PyLong_NumBits` and `_PyLong_AsByteArray` on PyPy. [#2146](https://github.com/PyO3/pyo3/pull/2146)
 - Fix memory leak in implementation of `AsPyPointer` for `Option<T>`. [#2160](https://github.com/PyO3/pyo3/pull/2160)
+- Fix the signature of `_PyLong_NumBits` [#2161](https://github.com/PyO3/pyo3/pull/2161)
 
 ## [0.15.1] - 2021-11-19
 
diff --git a/pyo3-ffi/src/longobject.rs b/pyo3-ffi/src/longobject.rs
index f44115cf2c4..9088fa3b431 100644
--- a/pyo3-ffi/src/longobject.rs
+++ b/pyo3-ffi/src/longobject.rs
@@ -97,7 +97,7 @@ extern "C" {
     // skipped non-limited _PyLong_Sign
 
     #[cfg_attr(PyPy, link_name = "_PyPyLong_NumBits")]
-    pub fn _PyLong_NumBits(obj: *mut PyObject) -> c_int;
+    pub fn _PyLong_NumBits(obj: *mut PyObject) -> size_t;
 
     // skipped _PyLong_DivmodNear
 
diff --git a/src/conversions/num_bigint.rs b/src/conversions/num_bigint.rs
index 197f7ec375b..f55dc3859f4 100644
--- a/src/conversions/num_bigint.rs
+++ b/src/conversions/num_bigint.rs
@@ -111,12 +111,12 @@ macro_rules! bigint_conversion {
                     let num: Py<PyLong> =
                         Py::from_owned_ptr_or_err(py, ffi::PyNumber_Index(ob.as_ptr()))?;
                     let n_bits = ffi::_PyLong_NumBits(num.as_ptr());
-                    let n_bytes = if n_bits == -1 {
+                    let n_bytes = if n_bits == (-1isize as usize) {
                         return Err(PyErr::fetch(py));
                     } else if n_bits == 0 {
                         0
                     } else {
-                        (n_bits as usize - 1 + $is_signed) / 8 + 1
+                        (n_bits - 1 + $is_signed) / 8 + 1
                     };
                     if n_bytes <= 128 {
                         let mut buffer = [0; 128];