diff --git a/compiler/rustc_const_eval/src/transform/validate.rs b/compiler/rustc_const_eval/src/transform/validate.rs
index c50e937d84f..3c350e25ba6 100644
--- a/compiler/rustc_const_eval/src/transform/validate.rs
+++ b/compiler/rustc_const_eval/src/transform/validate.rs
@@ -164,7 +164,7 @@ fn check_cleanup_control_flow(&self) {
                 if let Some(root) = post_contract_node.get(&bb) {
                     break *root;
                 }
-                let parent = doms.immediate_dominator(bb);
+                let parent = doms.immediate_dominator(bb).unwrap();
                 dom_path.push(bb);
                 if !self.body.basic_blocks[parent].is_cleanup {
                     break bb;
diff --git a/compiler/rustc_data_structures/src/graph/dominators/mod.rs b/compiler/rustc_data_structures/src/graph/dominators/mod.rs
index e76bdac2864..a7de709ba72 100644
--- a/compiler/rustc_data_structures/src/graph/dominators/mod.rs
+++ b/compiler/rustc_data_structures/src/graph/dominators/mod.rs
@@ -242,7 +242,9 @@ pub fn dominators<G: ControlFlowGraph>(graph: G) -> Dominators<G::Node> {
         immediate_dominators[*node] = Some(pre_order_to_real[idom[idx]]);
     }
 
-    Dominators { post_order_rank, immediate_dominators }
+    let start_node = graph.start_node();
+    immediate_dominators[start_node] = None;
+    Dominators { start_node, post_order_rank, immediate_dominators }
 }
 
 /// Evaluate the link-eval virtual forest, providing the currently minimum semi
@@ -308,6 +310,7 @@ fn compress(
 /// Tracks the list of dominators for each node.
 #[derive(Clone, Debug)]
 pub struct Dominators<N: Idx> {
+    start_node: N,
     post_order_rank: IndexVec<N, usize>,
     // Even though we track only the immediate dominator of each node, it's
     // possible to get its full list of dominators by looking up the dominator
@@ -316,14 +319,14 @@ pub struct Dominators<N: Idx> {
 }
 
 impl<Node: Idx> Dominators<Node> {
-    /// Whether the given Node has an immediate dominator.
+    /// Returns true if node is reachable from the start node.
     pub fn is_reachable(&self, node: Node) -> bool {
-        self.immediate_dominators[node].is_some()
+        node == self.start_node || self.immediate_dominators[node].is_some()
     }
 
-    pub fn immediate_dominator(&self, node: Node) -> Node {
-        assert!(self.is_reachable(node), "node {node:?} is not reachable");
-        self.immediate_dominators[node].unwrap()
+    /// Returns the immediate dominator of node, if any.
+    pub fn immediate_dominator(&self, node: Node) -> Option<Node> {
+        self.immediate_dominators[node]
     }
 
     /// Provides an iterator over each dominator up the CFG, for the given Node.
@@ -357,12 +360,7 @@ impl<'dom, Node: Idx> Iterator for Iter<'dom, Node> {
 
     fn next(&mut self) -> Option<Self::Item> {
         if let Some(node) = self.node {
-            let dom = self.dominators.immediate_dominator(node);
-            if dom == node {
-                self.node = None; // reached the root
-            } else {
-                self.node = Some(dom);
-            }
+            self.node = self.dominators.immediate_dominator(node);
             Some(node)
         } else {
             None
diff --git a/compiler/rustc_data_structures/src/graph/dominators/tests.rs b/compiler/rustc_data_structures/src/graph/dominators/tests.rs
index ff31d8f7fdc..8b124516623 100644
--- a/compiler/rustc_data_structures/src/graph/dominators/tests.rs
+++ b/compiler/rustc_data_structures/src/graph/dominators/tests.rs
@@ -8,7 +8,7 @@ fn diamond() {
 
     let dominators = dominators(&graph);
     let immediate_dominators = &dominators.immediate_dominators;
-    assert_eq!(immediate_dominators[0], Some(0));
+    assert_eq!(immediate_dominators[0], None);
     assert_eq!(immediate_dominators[1], Some(0));
     assert_eq!(immediate_dominators[2], Some(0));
     assert_eq!(immediate_dominators[3], Some(0));
@@ -30,7 +30,7 @@ fn paper() {
     assert_eq!(immediate_dominators[3], Some(6));
     assert_eq!(immediate_dominators[4], Some(6));
     assert_eq!(immediate_dominators[5], Some(6));
-    assert_eq!(immediate_dominators[6], Some(6));
+    assert_eq!(immediate_dominators[6], None);
 }
 
 #[test]
@@ -43,3 +43,13 @@ fn paper_slt() {
 
     dominators(&graph);
 }
+
+#[test]
+fn immediate_dominator() {
+    let graph = TestGraph::new(1, &[(1, 2), (2, 3)]);
+    let dominators = dominators(&graph);
+    assert_eq!(dominators.immediate_dominator(0), None);
+    assert_eq!(dominators.immediate_dominator(1), None);
+    assert_eq!(dominators.immediate_dominator(2), Some(1));
+    assert_eq!(dominators.immediate_dominator(3), Some(2));
+}