diff --git a/interactive/src/backend/vec.rs b/interactive/src/backend/vec.rs
index 8ef431415..fe9cea42e 100644
--- a/interactive/src/backend/vec.rs
+++ b/interactive/src/backend/vec.rs
@@ -182,7 +182,8 @@ impl Backend for VecBackend {
             Reducer::Distinct => Arc::new(|_key, _vals, output| { output.push((Value::unit(), 1)); }),
             // Count yields a one-field tuple `(count)`, keeping the convention
             // that a value is a tuple (so `$1[0]` and the explain envelope work).
-            Reducer::Count => Arc::new(|_key, vals, output| { let count: Diff = vals.iter().map(|(_, d)| *d).sum(); if count > 0 { output.push((Value::Tuple(vec![Value::Int(count)]), 1)); } }),
+            // The count is the sum of the diffs, of either sign, as DD's `count`.
+            Reducer::Count => Arc::new(|_key, vals, output| { let count: Diff = vals.iter().map(|(_, d)| *d).sum(); if count != 0 { output.push((Value::Tuple(vec![Value::Int(count)]), 1)); } }),
             // NEST: collect the key's values into a List, in value order (DD
             // hands them sorted), each repeated per its multiplicity.
             Reducer::Collect => Arc::new(|_key, vals, output| {
diff --git a/interactive/src/corgi/reduce.rs b/interactive/src/corgi/reduce.rs
index 378acf212..3dd14a7c2 100644
--- a/interactive/src/corgi/reduce.rs
+++ b/interactive/src/corgi/reduce.rs
@@ -305,12 +305,13 @@ where
         let mut out_ends: Vec<usize> = Vec::with_capacity(ends.len());
         match self.reducer {
             Reducer::Count => {
-                // Per-bracket sum of diffs; survivors become a `Tuple([Int(sum)])` = corgi `Prod([Int])`.
+                // Per-bracket sum of diffs, of either sign, as DD's `count`; a non-zero sum becomes a
+                // `Tuple([Int(sum)])` = corgi `Prod([Int])`.
                 let mut sums: Vec<i64> = Vec::new();
                 let mut start = 0;
                 for &end in ends {
                     let c: Diff = input[start..end].iter().map(|&(_, d)| d).sum();
-                    if c > 0 {
+                    if c != 0 {
                         sums.push(c);
                     }
                     out_ends.push(sums.len());
diff --git a/interactive/src/ir.rs b/interactive/src/ir.rs
index 2a529ecae..b4b1c153b 100644
--- a/interactive/src/ir.rs
+++ b/interactive/src/ir.rs
@@ -240,6 +240,8 @@ pub struct Projection { pub key: Term, pub val: Term }
 pub enum Reducer {
     Min,
     Distinct,
+    /// The sum of a key's diffs, as `(count)`, when it is not zero. It may be negative, as DD's
+    /// `count` is: a negative count is how a program finds a negative accumulation.
     Count,
     /// NEST: collect a key's values into a `Value::List`, in the values' own
     /// (`Value: Ord`) order — so deterministic, and position-ordered when the
diff --git a/interactive/tests/count.rs b/interactive/tests/count.rs
new file mode 100644
index 000000000..1aa7b7a66
--- /dev/null
+++ b/interactive/tests/count.rs
@@ -0,0 +1,39 @@
+//! `count` is the sum of a key's diffs, of either sign, as DD's `count`: a key whose values
+//! accumulate negatively has a negative count, and one whose values cancel has none.
+
+use interactive::ir::Value;
+use interactive::server::{evaluate, RenderBackend};
+use interactive::{lower, parse};
+
+const PROGRAM: &str = r#"
+    let pairs = input 0 : ((int, int) ; ()) | key($0[0] ; $0[1]);
+    let shifted = pairs | key($0 ; $1[0] + 1);
+    export "positive" = pairs | count;
+    export "negative" = pairs | negate | count;
+    export "mixed" = (pairs + pairs - shifted - shifted - shifted) | count;
+    export "cancel" = (pairs - shifted) | count;
+"#;
+
+fn ints(ns: &[i64]) -> Value { Value::Tuple(ns.iter().map(|&n| Value::Int(n)).collect()) }
+
+#[test]
+fn counts_carry_their_sign() {
+    let mut program = lower::lower_tree(parse::pipe::parse(PROGRAM));
+    program.optimize();
+    // Key 1 has two values, key 2 one.
+    let rows: Vec<(Value, Value)> = [[1, 10], [1, 20], [2, 30]].iter().map(|r| (ints(r), Value::unit())).collect();
+    let count = |k: i64, c: i64| ((ints(&[k]), ints(&[c])), 1);
+    let mut want = std::collections::BTreeMap::new();
+    want.insert("positive".to_string(), vec![count(1, 2), count(2, 1)]);
+    want.insert("negative".to_string(), vec![count(1, -2), count(2, -1)]);
+    // Each key: twice its values, less three times as many shifted ones.
+    want.insert("mixed".to_string(), vec![count(1, -2), count(2, -1)]);
+    // Each key's values cancel against as many shifted ones.
+    want.insert("cancel".to_string(), vec![]);
+    for backend in [RenderBackend::Vec, RenderBackend::Corgi] {
+        for workers in [1, 3] {
+            let got = evaluate(backend, timely::Config::process(workers), &program, &[rows.clone()]);
+            assert_eq!(got, want, "{backend:?}, {workers} workers");
+        }
+    }
+}