diff --git a/src/FsAutoComplete.Core/FileSystem.fs b/src/FsAutoComplete.Core/FileSystem.fs
index 0159ed758..5f4f48690 100644
--- a/src/FsAutoComplete.Core/FileSystem.fs
+++ b/src/FsAutoComplete.Core/FileSystem.fs
@@ -245,7 +245,12 @@ type NamedText(fileName: string<LocalPath>, str: string) =
   member x.TryGetPrevChar(pos: FSharp.Compiler.Text.Position) : (FSharp.Compiler.Text.Position * char) option =
     option {
       let! np = x.PrevPos pos
-      return np, x.GetCharUnsafe np
+      let! prevLineLength = x.GetLineLength(np)
+
+      if np.Column < 1 || prevLineLength < np.Column then
+        return! x.TryGetPrevChar(np)
+      else
+        return np, x.GetCharUnsafe np
     }
 
   /// split the TotalRange of this document at the range specified.
diff --git a/test/FsAutoComplete.Tests.Lsp/SignatureHelpTests.fs b/test/FsAutoComplete.Tests.Lsp/SignatureHelpTests.fs
index 2a51c01ad..8f54bd1f1 100644
--- a/test/FsAutoComplete.Tests.Lsp/SignatureHelpTests.fs
+++ b/test/FsAutoComplete.Tests.Lsp/SignatureHelpTests.fs
@@ -199,7 +199,11 @@ let issuesTests server =
         Expect.equal
           sigText
           "val count: x: int -> int"
-          "Should have no backticks because signatures are only ever rendered in `code` form") ]
+          "Should have no backticks because signatures are only ever rendered in `code` form")
+      testCaseAsync "issue #1040" // IndexOutOfRangeException
+        <| testSignatureHelp server "().ToString(\n\n,$0\n)" Manual (fun sigs ->
+          Expect.isSome sigs "Should have sigs"
+        ) ]
 
 let tests state =
   serverTestList "signature help" state defaultConfigDto None (fun server ->