diff --git a/.gitattributes b/.gitattributes
index 5bec94b3c3..02e9b84edd 100644
--- a/.gitattributes
+++ b/.gitattributes
@@ -16408,6 +16408,7 @@ tests/webtbs/tw33607.pp svneol=native#text/plain
 tests/webtbs/tw33635.pp svneol=native#text/pascal
 tests/webtbs/tw3364.pp svneol=native#text/plain
 tests/webtbs/tw3366.pp svneol=native#text/plain
+tests/webtbs/tw33666.pp svneol=native#text/plain
 tests/webtbs/tw33696.pp svneol=native#text/pascal
 tests/webtbs/tw33700.pp svneol=native#text/pascal
 tests/webtbs/tw33706.pp svneol=native#text/plain
diff --git a/compiler/ncon.pas b/compiler/ncon.pas
index ae94637c28..6a048420ff 100644
--- a/compiler/ncon.pas
+++ b/compiler/ncon.pas
@@ -982,6 +982,7 @@ implementation
                             Message1(option_code_page_not_available,IntToStr(cp1));
                           initwidestring(pw);
                           setlengthwidestring(pw,len);
+                          { returns room for terminating 0 }
                           l:=Utf8ToUnicode(PUnicodeChar(pw^.data),len,value_str,len);
                           if (l<>getlengthwidestring(pw)) then
                             begin
@@ -989,6 +990,7 @@ implementation
                               ReAllocMem(value_str,l);
                             end;
                           unicode2ascii(pw,value_str,cp1);
+                          len:=l-1;
                           donewidestring(pw);
                         end
                       else
@@ -1000,6 +1002,7 @@ implementation
                           initwidestring(pw);
                           setlengthwidestring(pw,len);
                           ascii2unicode(value_str,len,cp2,pw);
+                          { returns room for terminating 0 }
                           l:=UnicodeToUtf8(nil,0,PUnicodeChar(pw^.data),len);
                           if l<>len then
                             ReAllocMem(value_str,l);
diff --git a/tests/webtbs/tw33666.pp b/tests/webtbs/tw33666.pp
new file mode 100644
index 0000000000..9127dd476b
--- /dev/null
+++ b/tests/webtbs/tw33666.pp
@@ -0,0 +1,16 @@
+program Project1;
+
+{$mode objfpc}{$H+}
+{$Codepage UTF8}
+
+type
+  CP437String = type ansistring(437);
+
+var
+  s_cp437_1: CP437String;
+begin
+  s_cp437_1 := '║'; //<--- buggy
+  if (length(s_cp437_1)<> 1) or
+     (ord(s_cp437_1[1])<> 186) then
+    halt(1);
+end.