diff --git a/ide/sourcesyneditor.pas b/ide/sourcesyneditor.pas
index 736ce79eb1..1cc0a4c1eb 100644
--- a/ide/sourcesyneditor.pas
+++ b/ide/sourcesyneditor.pas
@@ -743,7 +743,8 @@ function TIDESynGutterMarks.SourceLineToDebugLine(aLinePos: Integer;
   AdjustOnError: Boolean): Integer;
 begin
   CheckTextBuffer;
-  if (aLinePos < 1) or (not HasDebugMarks) then exit(aLinePos);
+  if (aLinePos < 1) or (not HasDebugMarks) or (aLinePos >= FDebugMarkInfo.Count) then
+    exit(aLinePos);
   Result := FDebugMarkInfo[aLinePos - 1];
   while (Result = 0) and AdjustOnError and (aLinePos < FDebugMarkInfo.Count-1) do begin
     inc(aLinePos);