diff --git a/dialog.c b/dialog.c
index 88173aee..511331d3 100644
--- a/dialog.c
+++ b/dialog.c
@@ -392,8 +392,8 @@ dlgcontrol *ctrl_draglist(struct controlset *s, const char *label,
 }
 
 dlgcontrol *ctrl_filesel(struct controlset *s, const char *label,
-                         char shortcut, const char *filter, bool write,
-                         const char *title, HelpCtx helpctx,
+                         char shortcut, FILESELECT_FILTER_TYPE filter,
+                         bool write, const char *title, HelpCtx helpctx,
                          handler_fn handler, intorptr context)
 {
     dlgcontrol *c = ctrl_new(s, CTRL_FILESELECT, helpctx, handler, context);
diff --git a/dialog.h b/dialog.h
index ef9a9dfe..5ef20637 100644
--- a/dialog.h
+++ b/dialog.h
@@ -362,7 +362,7 @@ struct dlgcontrol {
              * particular platform might choose to cast integers into
              * this pointer type...
              */
-            char const *filter;
+            FILESELECT_FILTER_TYPE filter;
             /*
              * Some systems like to know whether a file selector is
              * choosing a file to read or one to write (and possibly
@@ -552,8 +552,8 @@ dlgcontrol *ctrl_draglist(struct controlset *, const char *label,
                           char shortcut, HelpCtx helpctx,
                           handler_fn handler, intorptr context);
 dlgcontrol *ctrl_filesel(struct controlset *, const char *label,
-                         char shortcut, const char *filter, bool write,
-                         const char *title, HelpCtx helpctx,
+                         char shortcut, FILESELECT_FILTER_TYPE filter,
+                         bool write, const char *title, HelpCtx helpctx,
                          handler_fn handler, intorptr context);
 dlgcontrol *ctrl_fontsel(struct controlset *, const char *label,
                          char shortcut, HelpCtx helpctx,
diff --git a/unix/platform.h b/unix/platform.h
index a75d70a6..51b89b2e 100644
--- a/unix/platform.h
+++ b/unix/platform.h
@@ -81,6 +81,8 @@ extern const struct BackendVtable pty_backend;
 typedef void *HelpCtx;
 #define NULL_HELPCTX ((HelpCtx)NULL)
 #define HELPCTX(x) NULL
+
+typedef const char *FILESELECT_FILTER_TYPE;
 #define FILTER_KEY_FILES NULL          /* FIXME */
 #define FILTER_DYNLIB_FILES NULL       /* FIXME */
 
diff --git a/windows/platform.h b/windows/platform.h
index 9463be4e..409aa824 100644
--- a/windows/platform.h
+++ b/windows/platform.h
@@ -286,6 +286,7 @@ void write_aclip(HWND hwnd, int clipboard, char *, int);
  * these strings are of exactly the type needed to go in
  * `lpstrFilter' in an OPENFILENAME structure.
  */
+typedef const char *FILESELECT_FILTER_TYPE;
 #define FILTER_KEY_FILES ("PuTTY Private Key Files (*.ppk)\0*.ppk\0" \
                               "All Files (*.*)\0*\0\0\0")
 #define FILTER_WAVE_FILES ("Wave Files (*.wav)\0*.WAV\0" \