diff --git a/src/toggl/mod.rs b/src/toggl/mod.rs
index 12f211a..89a3b9e 100644
--- a/src/toggl/mod.rs
+++ b/src/toggl/mod.rs
@@ -98,8 +98,8 @@ mod types {
         start: DateTime<Utc>,
         stop: Option<DateTime<Utc>>,
 
-        // TODO This should be an Option<u32> as all negatives signify currently running time entries
-        duration: i32,
+        #[serde(with = "duration_field")]
+        duration: Option<u32>,
 
         at: DateTime<Utc>,
 
@@ -115,6 +115,32 @@ mod types {
         server_deleted_at: Option<DateTime<Utc>>,
         permissions: Option<String>,
     }
+
+    mod duration_field {
+        use serde::{Deserialize, Serialize};
+
+        pub fn serialize<S>(duration: &Option<u32>, serializer: S) -> Result<S::Ok, S::Error>
+            where
+                S: serde::Serializer,
+        {
+            match duration {
+                None => i32::serialize(&-1, serializer),
+                Some(duration) => i32::serialize(&(*duration as i32), serializer),
+            }
+        }
+
+        pub fn deserialize<'de, D>(deserializer: D) -> Result<Option<u32>, D::Error>
+            where
+                D: serde::Deserializer<'de>,
+        {
+            let duration = i32::deserialize(deserializer)?;
+            if duration < 0 {
+                Ok(None)
+            } else {
+                Ok(Some(duration as u32))
+            }
+        }
+    }
 }
 
 #[derive(Debug, thiserror::Error)]