diff --git a/.gitattributes b/.gitattributes
index 29c7e4eaef..1e8cbc7380 100644
--- a/.gitattributes
+++ b/.gitattributes
@@ -9493,6 +9493,7 @@ tests/webtbf/tw14849.pp svneol=native#text/plain
 tests/webtbf/tw14929a.pp svneol=native#text/plain
 tests/webtbf/tw14929b.pp svneol=native#text/plain
 tests/webtbf/tw14946.pp svneol=native#text/plain
+tests/webtbf/tw15288.pp svneol=native#text/plain
 tests/webtbf/tw1599.pp svneol=native#text/plain
 tests/webtbf/tw1599b.pp svneol=native#text/plain
 tests/webtbf/tw1633.pp svneol=native#text/plain
diff --git a/compiler/ncnv.pas b/compiler/ncnv.pas
index a8c1dcc12f..6968d91c8f 100644
--- a/compiler/ncnv.pas
+++ b/compiler/ncnv.pas
@@ -515,9 +515,11 @@ implementation
 
                   stringdef :
                     begin
+                        if (p2.nodetype<>stringconstn) then
+                          Message(parser_e_illegal_expression)
                         { if we've already set elements which are constants }
                         { throw an error                                    }
-                        if ((hdef=nil) and assigned(buildp)) or
+                        else if ((hdef=nil) and assigned(buildp)) or
                           not(is_char(hdef)) then
                           CGMessage(type_e_typeconflict_in_set)
                         else
diff --git a/tests/webtbf/tw15288.pp b/tests/webtbf/tw15288.pp
new file mode 100644
index 0000000000..ec5271a22b
--- /dev/null
+++ b/tests/webtbf/tw15288.pp
@@ -0,0 +1,26 @@
+{ %fail }
+
+program setcrash;
+
+{$mode objfpc}{$H+}
+
+uses
+  Classes, SysUtils;
+
+var
+  AString: String='blabla';
+
+procedure GetKey(var Key: Char);
+begin
+  if Key in ['c', AString] then
+    writeln('OK');
+end;
+
+var
+  AKey: Char;
+begin
+  AKey := 'c';
+  GetKey(AKey);
+end.
+
+