diff --git a/projects/timelog.go b/projects/timelog.go
index 9c3b3fe..1a7589c 100644
--- a/projects/timelog.go
+++ b/projects/timelog.go
@@ -143,8 +143,9 @@ type TimelogCreateRequest struct {
 	// they will be summed up to calculate the total time logged.
 	Minutes int64 `json:"minutes"`
 
-	// Billable indicates whether the timelog is billable or not.
-	Billable bool `json:"isBillable"`
+	// Billable indicates whether the timelog is billable or not. When not
+	// provided, it will default to the tasklist or project settings.
+	Billable *bool `json:"isBillable"`
 
 	// UserID is an optional ID of the user who logged the timelog. If not
 	// provided, the timelog will be logged by the user making the request.
diff --git a/projects/timelog_test.go b/projects/timelog_test.go
index 5c88a31..25467a5 100644
--- a/projects/timelog_test.go
+++ b/projects/timelog_test.go
@@ -43,7 +43,7 @@ func TestTimelogCreate(t *testing.T) {
 			IsUTC:       true,
 			Hours:       2,
 			Minutes:     30,
-			Billable:    true,
+			Billable:    new(true),
 			UserID:      &testResources.UserID,
 			TagIDs:      []int64{testResources.TagID},
 		},
@@ -59,7 +59,7 @@ func TestTimelogCreate(t *testing.T) {
 			IsUTC:       true,
 			Hours:       2,
 			Minutes:     30,
-			Billable:    true,
+			Billable:    new(true),
 			UserID:      &testResources.UserID,
 			TagIDs:      []int64{testResources.TagID},
 		},