diff --git a/src/libkomwm.py b/src/libkomwm.py
index d69572b..c9381a4 100644
--- a/src/libkomwm.py
+++ b/src/libkomwm.py
@@ -3,6 +3,7 @@ from optparse import OptionParser
 import os
 import csv
 import functools
+from sys import exit
 from itertools import chain
 from multiprocessing import Pool, set_start_method
 from collections import OrderedDict
@@ -90,6 +91,8 @@ Priorities ranges' rendering order overview:
 - BG-by-size: landcover areas sorted by their size
 '''
 
+validation_errors_count = 0
+
 def to_boolean(s):
     s = s.lower()
     if s == "true" or s == "yes":
@@ -429,7 +432,9 @@ def get_drape_priority(cl, dr_type, object_id, auto_dr_type = None, auto_comment
                 prio_ranges[r]['priorities'][auto_prio_id] = priority
             return priority + prio_ranges[r]['base']
 
-    print(f'WARNING: priority is not set for {dr_type} {cl}{object_id}')
+    print(f'ERROR: priority is not set for {dr_type} {cl}{object_id}')
+    global validation_errors_count
+    validation_errors_count += 1
     return 0
 
 
@@ -850,6 +855,11 @@ def komap_mapswithme(options):
 
     validate_visibilities(options.maxzoom)
 
+    if validation_errors_count:
+        exit('FAILED to write regenerated drules files!\n'
+             f'There are {validation_errors_count} validation errors (see in the log above).\n'
+             'Fix all errors first and re-run.')
+
     output = ''
     for prio_range in prio_ranges.keys():
         dump_priorities(prio_range, options.priorities_path, options.maxzoom)