diff --git a/armi/nuclearDataIO/tests/test_xsLibraries.py b/armi/nuclearDataIO/tests/test_xsLibraries.py
index 1b65e7189..5b326d8d4 100644
--- a/armi/nuclearDataIO/tests/test_xsLibraries.py
+++ b/armi/nuclearDataIO/tests/test_xsLibraries.py
@@ -245,6 +245,24 @@ def test_mergeFailsWithNonIsotxsFiles(self):
         finally:
             os.remove(dummyFileName)
 
+        dummyFileName = "ISOtopics.txt"
+        with open(dummyFileName, "w") as file:
+            file.write(
+                "This is a file that starts with the letters 'ISO' but will"
+                " break the regular expression search."
+            )
+
+        try:
+            with mockRunLogs.BufferLog() as log:
+                lib = xsLibraries.IsotxsLibrary()
+                xsLibraries.mergeXSLibrariesInWorkingDirectory(lib)
+                self.assertTrue(
+                    f"Ignoring file {dummyFileName} in the merging of ISOXX files"
+                    in log.getStdoutValue()
+                )
+        finally:
+            os.remove(dummyFileName)
+
     def _xsLibraryAttributeHelper(
         self,
         lib,
diff --git a/armi/nuclearDataIO/xsLibraries.py b/armi/nuclearDataIO/xsLibraries.py
index a9fe19ca7..a634b40e1 100644
--- a/armi/nuclearDataIO/xsLibraries.py
+++ b/armi/nuclearDataIO/xsLibraries.py
@@ -175,9 +175,16 @@ def mergeXSLibrariesInWorkingDirectory(lib, xsLibrarySuffix="", mergeGammaLibs=F
     librariesToMerge = []
     neutronVelocities = {}  # Dictionary of neutron velocities from each ISOTXS file
     for xsLibFilePath in sorted(xsLibFiles):
-        xsID = re.search("ISO([A-Z0-9]{2})", xsLibFilePath).group(
-            1
-        )  # get XS ID from the cross section library name
+        try:
+            xsID = re.search("ISO([A-Z0-9]{2})", xsLibFilePath).group(
+                1
+            )  # get XS ID from the cross section library name
+        except AttributeError:
+            # if glob has matched something that is not actually an ISOXX file,
+            # the .group() call will fail
+            runLog.debug(f"Ignoring file {xsLibFilePath} in the merging of ISOXX files")
+            continue
+
         xsFileTypes = "ISOTXS" if not mergeGammaLibs else "ISOTXS, GAMISO, and PMATRX"
         runLog.info(
             "Retrieving {} data for XS ID {}{}".format(