diff --git a/mutannot.fsx b/mutannot.fsx
index a797846..821447f 100755
--- a/mutannot.fsx
+++ b/mutannot.fsx
@@ -178,27 +178,40 @@ let findMutation id mutations =
     |> Array.tryFind (fun mutation -> mutation.Id = id)
     |> Option.defaultWith (fun () -> fail $"Unknown mutation id: {id}")
 
-let lineNumberAt (text: string) index =
-    let mutable line = 1
-    for i in 0 .. index - 1 do
-        if text[i] = '\n' then
-            line <- line + 1
-    line
+let lineSpan (filePath: string) (text: string) (lineNumber: int) =
+    if lineNumber < 1 then
+        fail $"Line number must be positive: {lineNumber}"
 
-let replaceNearestOccurrence (mutation: MutationCase) (text: string) =
-    let rec collect fromIndex acc =
-        let idx = text.IndexOf(mutation.Find, fromIndex, StringComparison.Ordinal)
-        if idx < 0 then List.rev acc
-        else collect (idx + 1) (idx :: acc)
+    let mutable currentLine = 1
+    let mutable lineStart = 0
+    let mutable index = 0
 
-    let matches = collect 0 []
-    match matches with
-    | [] -> fail $"Could not find '{mutation.Find}' in {mutation.File}"
-    | _ ->
-        let chosen =
-            matches
-            |> List.minBy (fun idx -> abs (lineNumberAt text idx - mutation.Line))
-        text.Remove(chosen, mutation.Find.Length).Insert(chosen, mutation.Replace)
+    while index < text.Length && currentLine < lineNumber do
+        if text[index] = '\n' then
+            currentLine <- currentLine + 1
+            lineStart <- index + 1
+
+        index <- index + 1
+
+    if currentLine <> lineNumber then
+        fail $"Line {lineNumber} does not exist in {filePath}"
+
+    let mutable lineEnd = lineStart
+    while lineEnd < text.Length && text[lineEnd] <> '\n' && text[lineEnd] <> '\r' do
+        lineEnd <- lineEnd + 1
+
+    lineStart, lineEnd
+
+let replaceOccurrenceOnLine (mutation: MutationCase) (text: string) =
+    let lineStart, lineEnd = lineSpan mutation.File text mutation.Line
+    let lineText = text.Substring(lineStart, lineEnd - lineStart)
+    let lineOffset = lineText.IndexOf(mutation.Find, StringComparison.Ordinal)
+
+    if lineOffset < 0 then
+        fail $"Line {mutation.Line} in {mutation.File} does not contain '{mutation.Find}'."
+
+    let absoluteOffset = lineStart + lineOffset
+    text.Remove(absoluteOffset, mutation.Find.Length).Insert(absoluteOffset, mutation.Replace)
 
 let printMutation mutation =
     printfn "id: %s" mutation.Id
@@ -243,7 +256,7 @@ let runMutation (mutation: MutationCase) =
         fail $"Target file does not exist in worktree: {targetFile}"
 
     let originalText = File.ReadAllText targetFile
-    let mutatedText = replaceNearestOccurrence mutation originalText
+    let mutatedText = replaceOccurrenceOnLine mutation originalText
     File.WriteAllText(targetFile, mutatedText)
 
     printfn "==> %s: %s" mutation.Id mutation.TestName