diff --git a/src/conversion.rs b/src/conversion.rs
index bf3e7543..cfcd0f84 100644
--- a/src/conversion.rs
+++ b/src/conversion.rs
@@ -89,7 +89,7 @@ pub trait ToPyObject<'p> {
 ///
 /// In cases where the result does not depend on the `'prepared` lifetime,
 /// the inherent method `PyObject::extract()` can be used.
-pub trait ExtractPyObject<'python, 'source, 'prepared> {
+pub trait ExtractPyObject<'python, 'source, 'prepared> : Sized {
     type Prepared;
 
     fn prepare_extract(obj: &'source PyObject<'python>) -> PyResult<'python, Self::Prepared>;
@@ -97,8 +97,10 @@ pub trait ExtractPyObject<'python, 'source, 'prepared> {
     fn extract(prepared: &'prepared Self::Prepared) -> PyResult<'python, Self>;
 }
 
-impl <'python, 'source, 'prepared, T> ExtractPyObject<'python, 'source, 'prepared>
-    for T where T: PythonObjectWithCheckedDowncast<'python> {
+impl <'python, 'source, 'prepared, T> ExtractPyObject<'python, 'source, 'prepared> for T
+where T: PythonObjectWithCheckedDowncast<'python>,
+      'python: 'source
+{
 
     type Prepared = &'source PyObject<'python>;
 
@@ -108,7 +110,7 @@ impl <'python, 'source, 'prepared, T> ExtractPyObject<'python, 'source, 'prepare
     }
 
     #[inline]
-    fn extract(&&ref obj: &'prepared Self::Prepared) -> PyResult<'python, T> {
+    fn extract(&obj: &'prepared &'source PyObject<'python>) -> PyResult<'python, T> {
         Ok(try!(obj.clone().cast_into()))
     }
 }
diff --git a/src/objects/list.rs b/src/objects/list.rs
index 07e3182c..df272a68 100644
--- a/src/objects/list.rs
+++ b/src/objects/list.rs
@@ -137,7 +137,10 @@ impl <'p, T> ToPyObject<'p> for [T] where T: ToPyObject<'p> {
 }
 
 impl <'python, 'source, 'prepared, T> ExtractPyObject<'python, 'source, 'prepared>
-    for Vec<T> where T: for<'s, 'p> ExtractPyObject<'python, 's, 'p> {
+    for Vec<T>
+    where T: for<'s, 'p> ExtractPyObject<'python, 's, 'p>,
+          'python : 'source
+{
 
     type Prepared = &'source PyObject<'python>;
 
@@ -147,7 +150,7 @@ impl <'python, 'source, 'prepared, T> ExtractPyObject<'python, 'source, 'prepare
     }
 
     #[inline]
-    fn extract(&&ref obj: &'prepared Self::Prepared) -> PyResult<'python, Vec<T>> {
+    fn extract(&obj: &'prepared &'source PyObject<'python>) -> PyResult<'python, Vec<T>> {
         let list = try!(obj.cast_as::<PyList>());
         let mut v = Vec::with_capacity(list.len());
         for i in 0 .. list.len() {
diff --git a/src/objects/mod.rs b/src/objects/mod.rs
index e2c6d2da..bffe7b06 100644
--- a/src/objects/mod.rs
+++ b/src/objects/mod.rs
@@ -156,6 +156,7 @@ macro_rules! extract(
         impl <'python, 'source, 'prepared>
             ::conversion::ExtractPyObject<'python, 'source, 'prepared>
             for $t
+            where 'python: 'source
         {
 
             type Prepared = &'source PyObject<'python>;
diff --git a/src/objects/num.rs b/src/objects/num.rs
index c3e624fc..78fcd2f8 100644
--- a/src/objects/num.rs
+++ b/src/objects/num.rs
@@ -197,6 +197,7 @@ macro_rules! int_convert_u64_or_i64 (
 
         impl <'python, 'source, 'prepared>
             ExtractPyObject<'python, 'source, 'prepared> for $rust_type
+            where 'python: 'source
         {
             type Prepared = &'source PyObject<'python>;