diff --git a/prjxray/segmaker.py b/prjxray/segmaker.py
index 53ecc0ac..aa6c91b0 100644
--- a/prjxray/segmaker.py
+++ b/prjxray/segmaker.py
@@ -74,7 +74,8 @@ class Segmaker:
 
     def set_def_bt(self, block_type):
         '''Set default block type when more than one block present'''
-        assert block_type in BLOCK_TYPES
+        assert block_type in BLOCK_TYPES, (
+            "Unknown block type %r (known %r)" % (block_Type, BLOCK_TYPES))
         self.def_bt = block_type
 
     def load_grid(self):
@@ -239,7 +240,7 @@ class Segmaker:
                     elif re.match(r"SLICE_X[0-9]*[13579]Y", site):
                         return "SLICE_X1"
                     else:
-                        assert 0
+                        assert False, "Invalid name in %s" % site
 
                 def name_bram18():
                     # RAMB18_X0Y41
@@ -248,7 +249,7 @@ class Segmaker:
                     elif re.match(r"^RAMB18_X.*Y[0-9]*[13579]$", site):
                         return "RAMB18_Y1"
                     else:
-                        assert 0
+                        assert False, "Invalid name in %s" % site
 
                 def name_default():
                     # most sites are unique within their tile
@@ -326,7 +327,8 @@ class Segmaker:
             print("Tag tiles: %u" % (n_tile_tags, ))
             print("Used %u sites" % len(sites_used))
             print("Grid DB had %u tile types" % len(tile_types_found))
-        assert ntags == len(tags_used), "Unused tags"
+        assert ntags == len(tags_used), "Unused tags, %s used out of %s" % (
+            len(tag_used), ntags)
 
     def write(self, suffix=None, roi=False, allow_empty=False):
         assert self.segments_by_type, 'No data to write'