diff --git a/compiler/mono/src/decision_tree.rs b/compiler/mono/src/decision_tree.rs
index ae2de26840..ef6a508c54 100644
--- a/compiler/mono/src/decision_tree.rs
+++ b/compiler/mono/src/decision_tree.rs
@@ -1078,11 +1078,15 @@ pub fn optimize_when<'a>(
         .into_iter()
         .enumerate()
         .map(|(index, (pattern, guard, branch))| {
-            ((guard, pattern, index as u64), (index as u64, branch))
+            let has_guard = !guard.is_none();
+            (
+                (guard, pattern.clone(), index as u64),
+                (index as u64, branch, pattern, has_guard),
+            )
         })
         .unzip();
 
-    let indexed_branches: Vec<(u64, Stmt<'a>)> = _indexed_branches;
+    let indexed_branches: Vec<_> = _indexed_branches;
 
     let decision_tree = compile(patterns);
     let decider = tree_to_decider(decision_tree);
@@ -1094,7 +1098,14 @@ pub fn optimize_when<'a>(
     let mut choices = MutMap::default();
     let mut jumps = Vec::new();
 
-    for (index, branch) in indexed_branches.into_iter() {
+    for (index, mut branch, pattern, has_guard) in indexed_branches.into_iter() {
+        // bind the fields referenced in the pattern. For guards this happens separately, so
+        // the pattern variables are defined when evaluating the guard.
+        if !has_guard {
+            branch =
+                crate::ir::store_pattern(env, procs, layout_cache, &pattern, cond_symbol, branch);
+        }
+
         let ((branch_index, choice), opt_jump) = create_choices(&target_counts, index, branch);
 
         if let Some((index, body)) = opt_jump {
diff --git a/compiler/mono/src/ir.rs b/compiler/mono/src/ir.rs
index 55d1bc585e..a693052ab9 100644
--- a/compiler/mono/src/ir.rs
+++ b/compiler/mono/src/ir.rs
@@ -5096,8 +5096,6 @@ fn from_can_when<'a>(
                     jump,
                 );
 
-                // let new_guard_stmt = store_pattern(env, procs, layout_cache, &pattern, cond_symbol, guard_stmt);
-
                 (
                     pattern.clone(),
                     Guard::Guard {
@@ -5108,9 +5106,7 @@ fn from_can_when<'a>(
                     branch_stmt,
                 )
             } else {
-                let new_branch_stmt =
-                    store_pattern(env, procs, layout_cache, &pattern, cond_symbol, branch_stmt);
-                (pattern, Guard::NoGuard, new_branch_stmt)
+                (pattern, Guard::NoGuard, branch_stmt)
             }
         });
     let mono_branches = Vec::from_iter_in(it, arena);
diff --git a/compiler/test_mono/generated/nested_pattern_match.txt b/compiler/test_mono/generated/nested_pattern_match.txt
index 38b4165ff0..24279438b5 100644
--- a/compiler/test_mono/generated/nested_pattern_match.txt
+++ b/compiler/test_mono/generated/nested_pattern_match.txt
@@ -7,8 +7,8 @@ procedure Test.0 ():
     let Test.20 = Just Test.21;
     let Test.2 = Just Test.20;
     joinpoint Test.17:
-        let Test.11 = 1i64;
-        ret Test.11;
+        let Test.10 = 1i64;
+        ret Test.10;
     in
     let Test.15 = 0i64;
     let Test.16 = GetTagId Test.2;
@@ -19,8 +19,8 @@ procedure Test.0 ():
         let Test.14 = GetTagId Test.12;
         let Test.18 = lowlevel Eq Test.13 Test.14;
         if Test.18 then
-            let Test.10 = UnionAtIndex (Id 0) (Index 0) Test.2;
-            let Test.5 = UnionAtIndex (Id 0) (Index 0) Test.10;
+            let Test.11 = UnionAtIndex (Id 0) (Index 0) Test.2;
+            let Test.5 = UnionAtIndex (Id 0) (Index 0) Test.11;
             let Test.7 = 1i64;
             let Test.6 = CallByName Num.24 Test.5 Test.7;
             ret Test.6;
diff --git a/compiler/test_mono/generated/peano2.txt b/compiler/test_mono/generated/peano2.txt
index 673b61ffe4..4a1a5a234f 100644
--- a/compiler/test_mono/generated/peano2.txt
+++ b/compiler/test_mono/generated/peano2.txt
@@ -22,9 +22,9 @@ procedure Test.0 ():
             let Test.7 = 1i64;
             ret Test.7;
         else
-            let Test.9 = 0i64;
-            ret Test.9;
+            let Test.8 = 0i64;
+            ret Test.8;
     else
         dec Test.2;
-        let Test.10 = 0i64;
-        ret Test.10;
+        let Test.9 = 0i64;
+        ret Test.9;
diff --git a/compiler/test_mono/generated/quicksort_swap.txt b/compiler/test_mono/generated/quicksort_swap.txt
index fd927efcce..4845bb7c58 100644
--- a/compiler/test_mono/generated/quicksort_swap.txt
+++ b/compiler/test_mono/generated/quicksort_swap.txt
@@ -26,8 +26,8 @@ procedure Test.1 (Test.2):
     let Test.29 = CallByName List.3 Test.2 Test.31;
     let Test.8 = Struct {Test.29, Test.30};
     joinpoint Test.26:
-        let Test.19 = Array [];
-        ret Test.19;
+        let Test.17 = Array [];
+        ret Test.17;
     in
     let Test.23 = StructAtIndex 1 Test.8;
     let Test.24 = 1i64;
@@ -39,10 +39,10 @@ procedure Test.1 (Test.2):
         let Test.22 = GetTagId Test.20;
         let Test.27 = lowlevel Eq Test.21 Test.22;
         if Test.27 then
-            let Test.18 = StructAtIndex 0 Test.8;
-            let Test.4 = UnionAtIndex (Id 1) (Index 0) Test.18;
-            let Test.17 = StructAtIndex 1 Test.8;
-            let Test.5 = UnionAtIndex (Id 1) (Index 0) Test.17;
+            let Test.19 = StructAtIndex 0 Test.8;
+            let Test.4 = UnionAtIndex (Id 1) (Index 0) Test.19;
+            let Test.18 = StructAtIndex 1 Test.8;
+            let Test.5 = UnionAtIndex (Id 1) (Index 0) Test.18;
             let Test.16 = 0i64;
             let Test.10 = CallByName List.4 Test.2 Test.16 Test.5;
             let Test.11 = 0i64;
diff --git a/compiler/test_mono/generated/rigids.txt b/compiler/test_mono/generated/rigids.txt
index f8ccc8bde6..d6a146ba1b 100644
--- a/compiler/test_mono/generated/rigids.txt
+++ b/compiler/test_mono/generated/rigids.txt
@@ -24,8 +24,8 @@ procedure Test.1 (Test.2, Test.3, Test.4):
     let Test.32 = CallByName List.3 Test.4 Test.2;
     let Test.13 = Struct {Test.32, Test.33};
     joinpoint Test.29:
-        let Test.22 = Array [];
-        ret Test.22;
+        let Test.20 = Array [];
+        ret Test.20;
     in
     let Test.26 = StructAtIndex 1 Test.13;
     let Test.27 = 1i64;
@@ -37,10 +37,10 @@ procedure Test.1 (Test.2, Test.3, Test.4):
         let Test.25 = GetTagId Test.23;
         let Test.30 = lowlevel Eq Test.24 Test.25;
         if Test.30 then
-            let Test.21 = StructAtIndex 0 Test.13;
-            let Test.6 = UnionAtIndex (Id 1) (Index 0) Test.21;
-            let Test.20 = StructAtIndex 1 Test.13;
-            let Test.7 = UnionAtIndex (Id 1) (Index 0) Test.20;
+            let Test.22 = StructAtIndex 0 Test.13;
+            let Test.6 = UnionAtIndex (Id 1) (Index 0) Test.22;
+            let Test.21 = StructAtIndex 1 Test.13;
+            let Test.7 = UnionAtIndex (Id 1) (Index 0) Test.21;
             let Test.15 = CallByName List.4 Test.4 Test.2 Test.7;
             let Test.14 = CallByName List.4 Test.15 Test.3 Test.6;
             ret Test.14;
diff --git a/compiler/test_mono/generated/when_nested_maybe.txt b/compiler/test_mono/generated/when_nested_maybe.txt
index 38b4165ff0..24279438b5 100644
--- a/compiler/test_mono/generated/when_nested_maybe.txt
+++ b/compiler/test_mono/generated/when_nested_maybe.txt
@@ -7,8 +7,8 @@ procedure Test.0 ():
     let Test.20 = Just Test.21;
     let Test.2 = Just Test.20;
     joinpoint Test.17:
-        let Test.11 = 1i64;
-        ret Test.11;
+        let Test.10 = 1i64;
+        ret Test.10;
     in
     let Test.15 = 0i64;
     let Test.16 = GetTagId Test.2;
@@ -19,8 +19,8 @@ procedure Test.0 ():
         let Test.14 = GetTagId Test.12;
         let Test.18 = lowlevel Eq Test.13 Test.14;
         if Test.18 then
-            let Test.10 = UnionAtIndex (Id 0) (Index 0) Test.2;
-            let Test.5 = UnionAtIndex (Id 0) (Index 0) Test.10;
+            let Test.11 = UnionAtIndex (Id 0) (Index 0) Test.2;
+            let Test.5 = UnionAtIndex (Id 0) (Index 0) Test.11;
             let Test.7 = 1i64;
             let Test.6 = CallByName Num.24 Test.5 Test.7;
             ret Test.6;