diff --git a/src/functions.rs b/src/functions.rs
index e2048c2..a36b6fa 100644
--- a/src/functions.rs
+++ b/src/functions.rs
@@ -336,7 +336,7 @@ pub trait Aggregate<A, T> where T: ToResult {
     /// Initializes the aggregation context.
     fn init(&self) -> A; // TODO Validate: Fn(&Context)
     /// "step" function called once for each row in an aggregate group.
-    fn step(&self, &mut Context, &mut A); // TODO Validate: Fn(&Context, A) -> A
+    fn step(&self, &mut Context, &mut A) -> Result<()>; // TODO Validate: Fn(&Context, A) -> A
     /// Computes and sets the final result.
     fn finalize(&self, &A) -> Result<T>;
 }
@@ -441,7 +441,7 @@ impl InnerConnection {
                     if let Some(Ok(cstr)) = s.map(|s| str_to_cstring(&s)) {
                         ffi::sqlite3_result_error(ctx.ctx, cstr.as_ptr(), -1);
                     }
-                },
+                }
                 Err(err) => {
                     ffi::sqlite3_result_error_code(ctx.ctx, ffi::SQLITE_CONSTRAINT_FUNCTION);
                     if let Ok(cstr) = str_to_cstring(err.description()) {
@@ -498,7 +498,8 @@ impl InnerConnection {
             }
             let ac: *mut A = if (*pac).is_null() {
                 let a = (*boxed_aggr).init();
-                Box::into_raw(Box::new(a))
+                *pac = Box::into_raw(Box::new(a));
+                *pac
             } else {
                 *pac
             };
@@ -508,7 +509,21 @@ impl InnerConnection {
                 args: slice::from_raw_parts(argv, argc as usize),
             };
 
-            (*boxed_aggr).step(&mut ctx, &mut *ac);
+            match (*boxed_aggr).step(&mut ctx, &mut *ac) {
+                Ok(_) => {}
+                Err(Error::SqliteFailure(err, s)) => {
+                    ffi::sqlite3_result_error_code(ctx.ctx, err.extended_code);
+                    if let Some(Ok(cstr)) = s.map(|s| str_to_cstring(&s)) {
+                        ffi::sqlite3_result_error(ctx.ctx, cstr.as_ptr(), -1);
+                    }
+                }
+                Err(err) => {
+                    ffi::sqlite3_result_error_code(ctx.ctx, ffi::SQLITE_CONSTRAINT_FUNCTION);
+                    if let Ok(cstr) = str_to_cstring(err.description()) {
+                        ffi::sqlite3_result_error(ctx.ctx, cstr.as_ptr(), -1);
+                    }
+                }
+            };
         }
         unsafe extern "C" fn call_boxed_final<A, D, T>(ctx: *mut sqlite3_context)
             where D: Aggregate<A, T>,
@@ -523,16 +538,23 @@ impl InnerConnection {
                 return;
             }
             let ac: *mut A = *pac;
+            let a = Box::from_raw(mem::transmute(ac)); // to be freed
 
-            match (*boxed_aggr).finalize(&*ac) {
+            match (*boxed_aggr).finalize(&a) {
                 Ok(r) => r.set_result(ctx),
-                Err(e) => {
-                    ffi::sqlite3_result_error_code(ctx, e.code);
-                    if let Ok(cstr) = str_to_cstring(&e.message) {
+                Err(Error::SqliteFailure(err, s)) => {
+                    ffi::sqlite3_result_error_code(ctx, err.extended_code);
+                    if let Some(Ok(cstr)) = s.map(|s| str_to_cstring(&s)) {
                         ffi::sqlite3_result_error(ctx, cstr.as_ptr(), -1);
                     }
                 }
-            }
+                Err(err) => {
+                    ffi::sqlite3_result_error_code(ctx, ffi::SQLITE_CONSTRAINT_FUNCTION);
+                    if let Ok(cstr) = str_to_cstring(err.description()) {
+                        ffi::sqlite3_result_error(ctx, cstr.as_ptr(), -1);
+                    }
+                }
+            };
         }
 
         let boxed_aggr: *mut D = Box::into_raw(Box::new(aggr));
@@ -581,7 +603,7 @@ mod test {
     use self::regex::Regex;
 
     use {Connection, Error, Result};
-    use functions::Context;
+    use functions::{Aggregate, Context};
 
     fn half(ctx: &Context) -> Result<c_double> {
         assert!(ctx.len() == 1, "called with unexpected number of arguments");
@@ -739,4 +761,42 @@ mod test {
             assert_eq!(expected, result);
         }
     }
+
+    struct Sum;
+
+    impl Aggregate<i64, i64> for Sum {
+        fn init(&self) -> i64 {
+            0
+        }
+
+        fn step(&self, ctx: &mut Context, sum: &mut i64) -> Result<()> {
+            *sum = *sum + try!(ctx.get::<i64>(0));
+            Ok(())
+        }
+
+        fn finalize(&self, sum: &i64) -> Result<i64> {
+            Ok(*sum)
+        }
+    }
+
+    #[test]
+    fn test_sum() {
+        let db = Connection::open_in_memory().unwrap();
+        db.create_aggregate_function("my_sum", 1, true, Sum).unwrap();
+        let no_result = "SELECT my_sum(i) FROM (SELECT 2 AS i WHERE 1 <> 1)";
+        let result: Option<i64> = db.query_row(no_result, &[], |r| r.get(0))
+                                    .unwrap();
+        assert!(result.is_none());
+
+        let single_sum = "SELECT my_sum(i) FROM (SELECT 2 AS i UNION ALL SELECT 2)";
+        let result: i64 = db.query_row(single_sum, &[], |r| r.get(0))
+                            .unwrap();
+        assert_eq!(4, result);
+
+        let single_sum = "SELECT my_sum(i), my_sum(j) FROM (SELECT 2 AS i, 1 AS j UNION ALL \
+                          SELECT 2, 1)";
+        let result: (i64, i64) = db.query_row(single_sum, &[], |r| (r.get(0), r.get(1)))
+                                   .unwrap();
+        assert_eq!((4, 2), result);
+    }
 }