diff --git a/crates/kernel/src/iges_in/tests.rs b/crates/kernel/src/iges_in/tests.rs
index 4abed67300..9a120751ef 100644
--- a/crates/kernel/src/iges_in/tests.rs
+++ b/crates/kernel/src/iges_in/tests.rs
@@ -361,6 +361,54 @@ fn surfaces_in_separate_groups_remain_separate_parts() {
     }
 }
 
+/// The box of [`trimmed_surfaces_sew_into_a_solid`] with its edge (0, 0, 0)–(0, 20, 0) stopping
+/// 0.01 mm short of its corner in both faces that use it, its next pieces starting at the
+/// corner. The pieces of a trim curve run on from one another, so the edge ends at the corner the
+/// other three edges there agree on, exactly: a closed box SMLib finds valid. The edge was left
+/// short with the gap on the vertex's tolerance, 2000 times the file's resolution (#3845).
+#[test]
+fn trim_pieces_that_miss_their_corner_are_closed_onto_it() {
+    let c =
+        |i: usize| -> [f64; 3] { [if i & 1 != 0 { 10.0 } else { 0.0 }, if i & 2 != 0 { 20.0 } else { 0.0 }, if i & 4 != 0 { 30.0 } else { 0.0 }] };
+    let short = [0.0, 19.99, 0.0];
+    let quads = [[0, 2, 3, 1], [4, 5, 7, 6], [0, 1, 5, 4], [2, 6, 7, 3], [0, 4, 6, 2], [1, 3, 7, 5]];
+    let mut ents: Vec<(i64, i64, String)> = Vec::new();
+    let fmt = |p: [f64; 3]| format!("{:?},{:?},{:?}", p[0], p[1], p[2]);
+    for q in quads {
+        let (a, b, cc, d) = (c(q[0]), c(q[1]), c(q[2]), c(q[3]));
+        let surf = format!("1,1,1,1,0,0,1,0,0,0.,0.,1.,1.,0.,0.,1.,1.,1.,1.,1.,1.,{},{},{},{},0.,1.,0.,1.", fmt(a), fmt(b), fmt(d), fmt(cc));
+        ents.push((128, 0, surf));
+        let s_de = 2 * ents.len() - 1;
+        let mut lines = Vec::new();
+        for (p0, p1) in [(a, b), (b, cc), (cc, d), (d, a)] {
+            // The short edge: its end at (0, 20, 0) is 0.01 short.
+            let (p0, p1) = match (p0, p1) {
+                ([0.0, 0.0, 0.0], [0.0, 20.0, 0.0]) => (p0, short),
+                ([0.0, 20.0, 0.0], [0.0, 0.0, 0.0]) => (short, p1),
+                x => x,
+            };
+            ents.push((110, 0, format!("{},{}", fmt(p0), fmt(p1))));
+            lines.push(2 * ents.len() - 1);
+        }
+        ents.push((102, 0, format!("4,{}", lines.iter().map(|x| x.to_string()).collect::<Vec<_>>().join(","))));
+        let comp = 2 * ents.len() - 1;
+        ents.push((142, 0, format!("0,{s_de},0,{comp},2")));
+        let cos = 2 * ents.len() - 1;
+        ents.push((144, 0, format!("{s_de},1,0,{cos}")));
+    }
+    let imp = iges_import(&iges_file(&ents)).unwrap();
+    assert!(imp.warnings.is_empty(), "{:?}", imp.warnings);
+    assert!(imp.bodies[0].closed);
+    let body = &imp.bodies[0].body;
+    let check = body.smlib().unwrap().check(solidcraft_smlib::CheckLevel::All).unwrap();
+    assert!(check.is_valid(), "{check}");
+    for v in &body.record().vertices {
+        assert!(v.tolerance <= body.record().tolerance, "a vertex {:?} kept a gap in its tolerance {}", v.point, v.tolerance);
+    }
+    let m = measure(body).unwrap();
+    assert!(rel(m.volume, 6000.0) < 1e-6 && rel(m.area, 2200.0) < 1e-6, "{} {}", m.volume, m.area);
+}
+
 #[test]
 fn a_drill_point_round_trips_through_iges() {
     // A blind hole with a cone tip: the tip's face closes at its apex (its seam runs out to the