diff --git a/ide/sourcesyneditor.pas b/ide/sourcesyneditor.pas
index d6f7ea234b..f65c77fc9f 100644
--- a/ide/sourcesyneditor.pas
+++ b/ide/sourcesyneditor.pas
@@ -235,9 +235,13 @@ begin
       FDebugMarkInfo.IncRefCount;
   end;
 
-  if ALastLinePos >= FDebugMarkInfo.Count then begin
+  if ALastLinePos > FDebugMarkInfo.Count then begin
     debugln(['Request to set debug-mark out of range: max-count=',FDebugMarkInfo.Count,' Marks=',AFirstLinePos,' to=',ALastLinePos]);
-    ALastLinePos := FDebugMarkInfo.Count  -1;
+    ALastLinePos := FDebugMarkInfo.Count;
+  end;
+  if AFirstLinePos < 1 then begin
+    debugln(['Request to set debug-mark out of range: max-count=',FDebugMarkInfo.Count,' Marks=',AFirstLinePos,' to=',ALastLinePos]);
+    AFirstLinePos := 1;
   end;
   for i := AFirstLinePos - 1 to ALastLinePos - 1 do
     FDebugMarkInfo[i] := i + 1;