diff --git a/fuzzers/035b-iob-iserdes/generate.py b/fuzzers/035b-iob-iserdes/generate.py
index daa3469a..63784373 100644
--- a/fuzzers/035b-iob-iserdes/generate.py
+++ b/fuzzers/035b-iob-iserdes/generate.py
@@ -64,10 +64,10 @@ def run():
                     if i == "NETWORKING":
                         for j in data_rates:
                             for k in data_widths[j]:
-                                tag = "ISERDES.%s.%s.%s" % (i, j, k)
+                                tag = "ISERDES.%s.%s.W%s" % (i, j, k)
                                 segmk.add_site_tag(loc, tag, 0)
                     else:
-                        segmk.add_site_tag(loc, "ISERDES.%s.DDR.4" % i, 0)
+                        segmk.add_site_tag(loc, "ISERDES.%s.DDR.W4" % i, 0)
 
                 segmk.add_site_tag(loc, "ISERDES.NUM_CE.N1", 0)
                 segmk.add_site_tag(loc, "ISERDES.NUM_CE.N2", 0)
@@ -120,7 +120,7 @@ def run():
                     if i == "NETWORKING":
                         for j in data_rates:
                             for k in data_widths[j]:
-                                tag = "ISERDES.%s.%s.%s" % (i, j, k)
+                                tag = "ISERDES.%s.%s.W%s" % (i, j, k)
                                 val = 0
 
                                 if i == iface_type:
@@ -129,7 +129,7 @@ def run():
                                             segmk.add_site_tag(loc, tag, 1)
                     else:
                         if i == iface_type:
-                            segmk.add_site_tag(loc, "ISERDES.%s.DDR.4" % i, 1)
+                            segmk.add_site_tag(loc, "ISERDES.%s.DDR.W4" % i, 1)
 
                 if "NUM_CE" in params:
                     value = params["NUM_CE"]
diff --git a/utils/mergedb.py b/utils/mergedb.py
index f3d477a3..4aa412ef 100755
--- a/utils/mergedb.py
+++ b/utils/mergedb.py
@@ -9,9 +9,39 @@
 #
 # SPDX-License-Identifier: ISC
 
-import os, sys
+import os, sys, re
 from prjxray import util
 
+TAG_PART_RE = re.compile(r"^[a-zA-Z][0-9a-zA-Z_]*(\[[0-9]+\])?$")
+
+
+def check_tag_name(tag):
+    '''
+    Checks if the tag name given by the used conforms to the valid fasm
+    name rules.
+
+    >>> check_tag_name("CELL.feature19.ENABLED")
+    True
+    >>> check_tag_name("FEATURE")
+    True
+    >>> check_tag_name("TAG.")
+    False
+    >>> check_tag_name(".TAG")
+    False
+    >>> check_tag_name("CELL..FEATURE")
+    False
+    >>> check_tag_name("CELL.3ENABLE")
+    False
+    >>> check_tag_name("FEATURE.12.ON")
+    False
+    '''
+
+    for part in tag.split("."):
+        if not len(part) or TAG_PART_RE.match(part) is None:
+            return False
+
+    return True
+
 
 def run(fn_ins, fn_out, strict=False, track_origin=False, verbose=False):
     # tag to bits
@@ -27,6 +57,9 @@ def run(fn_ins, fn_out, strict=False, track_origin=False, verbose=False):
             assert mode is not None or mode != "always", "strict: got ill defined line: %s" % (
                 line, )
 
+            if not check_tag_name(tag):
+                assert not strict, "strict: Invalid tag name '{}'".format(tag)
+
             if tag in tags:
                 orig_bits, orig_line, orig_origin = tags[tag]
                 if orig_bits != bits: