Index: macosx/tkMacOSXWm.c
==================================================================
--- macosx/tkMacOSXWm.c
+++ macosx/tkMacOSXWm.c
@@ -3428,39 +3428,46 @@
 	wmPtr->masterWindowName = NULL;
     } else {
 	if (TkGetWindowFromObj(interp, tkwin, objv[3], &master) != TCL_OK) {
 	    return TCL_ERROR;
 	}
-	Tk_MakeWindowExist(master);
+	TkWindow* masterPtr = (TkWindow*) master;
+	while (!Tk_TopWinHierarchy(masterPtr)) {
+            /*
+             * Ensure that the master window is actually a Tk toplevel.
+             */
+
+            masterPtr = masterPtr->parentPtr;
+        }
+	Tk_MakeWindowExist((Tk_Window)masterPtr);
 
 	if (wmPtr->iconFor != NULL) {
 	    Tcl_SetObjResult(interp, Tcl_ObjPrintf(
 		    "can't make \"%s\" a transient: it is an icon for %s",
 		    Tcl_GetString(objv[2]), Tk_PathName(wmPtr->iconFor)));
 	    Tcl_SetErrorCode(interp, "TK", "WM", "TRANSIENT", "ICON", NULL);
 	    return TCL_ERROR;
 	}
 
-	wmPtr2 = ((TkWindow *) master)->wmInfoPtr;
-
+	wmPtr2 = masterPtr->wmInfoPtr;
 	/* Under some circumstances, wmPtr2 is NULL here */
 	if (wmPtr2 != NULL && wmPtr2->iconFor != NULL) {
 	    Tcl_SetObjResult(interp, Tcl_ObjPrintf(
 		    "can't make \"%s\" a master: it is an icon for %s",
 		    Tcl_GetString(objv[3]), Tk_PathName(wmPtr2->iconFor)));
 	    Tcl_SetErrorCode(interp, "TK", "WM", "TRANSIENT", "ICON", NULL);
 	    return TCL_ERROR;
 	}
 
-	if ((TkWindow *) master == winPtr) {
+	if (masterPtr == winPtr) {
 	    Tcl_SetObjResult(interp, Tcl_ObjPrintf(
 		    "can't make \"%s\" its own master", Tk_PathName(winPtr)));
 	    Tcl_SetErrorCode(interp, "TK", "WM", "TRANSIENT", "SELF", NULL);
 	    return TCL_ERROR;
 	}
 
-	wmPtr->master = Tk_WindowId(master);
+	wmPtr->master = Tk_WindowId(masterPtr);
 	masterWindowName = Tcl_GetStringFromObj(objv[3], &length);
 	if (wmPtr->masterWindowName != NULL) {
 	    ckfree(wmPtr->masterWindowName);
 	}
 	wmPtr->masterWindowName = ckalloc(length+1);