diff --git a/xwords4/linux/cursesmain.c b/xwords4/linux/cursesmain.c
index a74f242ca..96d862fc7 100644
--- a/xwords4/linux/cursesmain.c
+++ b/xwords4/linux/cursesmain.c
@@ -204,7 +204,7 @@ static CursesMenuHandler getHandlerForKey( const MenuList* list, char ch );
 /* extern int errno; */
 
 static void
-cursesUserError( CursesAppGlobals* globals, char* format, ... )
+cursesUserError( CursesAppGlobals* globals, const char* format, ... )
 {
     char buf[512];
     va_list ap;
@@ -241,7 +241,7 @@ curses_util_userError( XW_UtilCtxt* uc, UtilErrID id )
 {
     CursesAppGlobals* globals = (CursesAppGlobals*)uc->closure;
     XP_Bool silent;
-    XP_UCHAR* message = linux_getErrString( id, &silent );
+    const XP_UCHAR* message = linux_getErrString( id, &silent );
 
     if ( silent ) {
         XP_LOGF( "silent userError: %s", message );
diff --git a/xwords4/linux/gtkmain.c b/xwords4/linux/gtkmain.c
index 9dc824f54..c8bf1ddd8 100644
--- a/xwords4/linux/gtkmain.c
+++ b/xwords4/linux/gtkmain.c
@@ -1059,7 +1059,7 @@ handle_commit_button( GtkWidget* XP_UNUSED(widget), GtkAppGlobals* globals )
 } /* handle_commit_button */
 
 static void
-gtkUserError( GtkAppGlobals* XP_UNUSED(globals), char* format, ... )
+gtkUserError( GtkAppGlobals* XP_UNUSED(globals), const char* format, ... )
 {
     char buf[512];
     va_list ap;
@@ -1412,7 +1412,7 @@ gtk_util_userError( XW_UtilCtxt* uc, UtilErrID id )
 {
     GtkAppGlobals* globals = (GtkAppGlobals*)uc->closure;
     XP_Bool silent;
-    XP_UCHAR* message = linux_getErrString( id, &silent );
+    const XP_UCHAR* message = linux_getErrString( id, &silent );
 
     XP_LOGF( "%s(%d)", __func__, id );
 
diff --git a/xwords4/linux/linuxmain.c b/xwords4/linux/linuxmain.c
index 982f26501..e7b168d80 100644
--- a/xwords4/linux/linuxmain.c
+++ b/xwords4/linux/linuxmain.c
@@ -135,11 +135,12 @@ strFromStream( XWStreamCtxt* stream )
     return buf;
 } /* strFromStream */
 
-XP_UCHAR*
+
+const XP_UCHAR*
 linux_getErrString( UtilErrID id, XP_Bool* silent )
 {
     *silent = XP_FALSE;
-    char* message = NULL;
+    const char* message = NULL;
 
     switch( id ) {
     case ERR_TILES_NOT_IN_LINE:
@@ -184,6 +185,10 @@ linux_getErrString( UtilErrID id, XP_Bool* silent )
         break;
 #endif
 
+    case ERR_CANT_TRADE_MID_MOVE:
+        message = "Remove played tiles before trading.";
+        break;
+
     case ERR_CANT_UNDO_TILEASSIGN:
         message = "Tile assignment can't be undone.";
         break;
diff --git a/xwords4/linux/linuxmain.h b/xwords4/linux/linuxmain.h
index c8bd53ccf..4b0a623ea 100644
--- a/xwords4/linux/linuxmain.h
+++ b/xwords4/linux/linuxmain.h
@@ -56,7 +56,7 @@ void linuxFireTimer( CommonGlobals* cGlobals, XWTimerReason why );
 
 XWStreamCtxt* stream_from_msgbuf( CommonGlobals* cGlobals, 
                                   unsigned char* bufPtr, XP_U16 nBytes );
-XP_UCHAR* linux_getErrString( UtilErrID id, XP_Bool* silent );
+const XP_UCHAR* linux_getErrString( UtilErrID id, XP_Bool* silent );
 XP_UCHAR* strFromStream( XWStreamCtxt* stream );
 
 void catGameHistory( CommonGlobals* cGlobals );