diff --git a/.gitattributes b/.gitattributes
index 7f4a3d7925..e52ed0f0e6 100644
--- a/.gitattributes
+++ b/.gitattributes
@@ -12567,6 +12567,7 @@ tests/webtbs/tw2178.pp svneol=native#text/plain
 tests/webtbs/tw2185.pp svneol=native#text/plain
 tests/webtbs/tw2186.pp svneol=native#text/plain
 tests/webtbs/tw2187.pp svneol=native#text/plain
+tests/webtbs/tw21878.pp svneol=native#text/plain
 tests/webtbs/tw2196.pp svneol=native#text/plain
 tests/webtbs/tw2197.pp svneol=native#text/plain
 tests/webtbs/tw2198.pp svneol=native#text/plain
diff --git a/compiler/ninl.pas b/compiler/ninl.pas
index 0b401bc650..fb28463dcd 100644
--- a/compiler/ninl.pas
+++ b/compiler/ninl.pas
@@ -1599,8 +1599,10 @@ implementation
         while assigned(ppn.right) do
          begin
            inc(counter);
+           set_varstate(ppn.left,vs_read,[vsf_must_be_valid]);
            ppn:=tcallparanode(ppn.right);
          end;
+        set_varstate(ppn.left,vs_read,[vsf_must_be_valid]);
         paradef:=ppn.left.resultdef;
         if is_ansistring(paradef) then
           // set resultdef to argument def
diff --git a/tests/webtbs/tw21878.pp b/tests/webtbs/tw21878.pp
new file mode 100644
index 0000000000..e2fca6a58c
--- /dev/null
+++ b/tests/webtbs/tw21878.pp
@@ -0,0 +1,23 @@
+{ %norun }
+{ %opt=-vn -Sen }
+
+{$mode objfpc}{$H+}
+
+uses
+  Classes, SysUtils;
+
+function DoSomething(const AName: string): integer;
+var
+  i, l: Integer;
+begin
+  Result := 0;;
+  l := Length(AName);
+  for i:= 1 to Paramcount do begin
+    if copy(ParamStr(i),1, l) = AName then
+      inc(Result);
+  end;
+end;
+
+begin
+  DoSomething('a');
+end.