diff --git a/Sources/_StringProcessing/Utility/AsciiBitset.swift b/Sources/_StringProcessing/Utility/AsciiBitset.swift
index 61a6a6891..67f5f3937 100644
--- a/Sources/_StringProcessing/Utility/AsciiBitset.swift
+++ b/Sources/_StringProcessing/Utility/AsciiBitset.swift
@@ -46,7 +46,7 @@ extension DSLTree.CustomCharacterClass {
       setBit(val)
       if isCaseInsensitive {
         switch val {
-          case 64...90: setBit(val + 32)
+          case 65...90: setBit(val + 32)
           case 97...122: setBit(val - 32)
           default: break
         }
diff --git a/Tests/RegexTests/MatchTests.swift b/Tests/RegexTests/MatchTests.swift
index d75dbde64..2d9a6b1c2 100644
--- a/Tests/RegexTests/MatchTests.swift
+++ b/Tests/RegexTests/MatchTests.swift
@@ -1458,6 +1458,29 @@ extension RegexTests {
         firstMatchTest(
           "(?i)[X-cD]", input: input, match: input, semanticLevel: semantics)
       }
+
+      // Boundary tests for case insensitive ASCII matching.
+      firstMatchTest(
+        "(?i)[@]", input: "@", match: "@", semanticLevel: semantics)
+      firstMatchTest(
+        "(?i)[@]", input: "`", match: nil, semanticLevel: semantics)
+      firstMatchTest(
+        "(?i)[`]", input: "`", match: "`", semanticLevel: semantics)
+      firstMatchTest(
+        "(?i)[`]", input: "@", match: nil, semanticLevel: semantics)
+      // A range that spans the "@"/"A" boundary folds "A" but not "@".
+      firstMatchTest(
+        "(?i)[?-A]", input: "a", match: "a", semanticLevel: semantics)
+      firstMatchTest(
+        "(?i)[?-A]", input: "`", match: nil, semanticLevel: semantics)
+      // The letter ends of each range still fold.
+      for (pattern, input) in [
+        ("(?i)[A]", "a"), ("(?i)[a]", "A"),
+        ("(?i)[Z]", "z"), ("(?i)[z]", "Z"),
+      ] {
+        firstMatchTest(
+          pattern, input: input, match: input, semanticLevel: semantics)
+      }
     }
   }