diff --git a/htsinfer/get_library_type.py b/htsinfer/get_library_type.py
index ca68c4e..19665c9 100644
--- a/htsinfer/get_library_type.py
+++ b/htsinfer/get_library_type.py
@@ -206,7 +206,13 @@ def _align_mates(self):
     def _update_relationship(self, concordant, read_counter):
         """Helper function to update relationship based on alignment."""
         try:
-            if (concordant / read_counter) >= self.cutoff:
+            ratio = concordant / read_counter
+        except ZeroDivisionError:
+            self.results.relationship = (
+                StatesTypeRelationship.not_available
+            )
+        else:
+            if ratio >= self.cutoff:
                 self.results.relationship = (
                     StatesTypeRelationship.split_mates
                 )
@@ -218,10 +224,6 @@ def _update_relationship(self, concordant, read_counter):
                 self.results.relationship = (
                     StatesTypeRelationship.not_mates
                 )
-        except ZeroDivisionError:
-            self.results.relationship = (
-                StatesTypeRelationship.not_available
-            )
 
     class AlignedSegment:
         """Placeholder class for mypy "Missing attribute"
@@ -331,7 +333,7 @@ def evaluate(self) -> None:
 
                 if self.seq_id_format is None:
                     self.result = StatesType.not_available
-                    LOGGER.warning(
+                    LOGGER.debug(
                         "Could not determine sequence identifier format."
                     )
                 else:
diff --git a/tests/test_get_library_type.py b/tests/test_get_library_type.py
index 9682bf7..2ae8c96 100644
--- a/tests/test_get_library_type.py
+++ b/tests/test_get_library_type.py
@@ -33,7 +33,6 @@
     FILE_IDS_NOT_MATCH_2,
     FILE_TRANSCRIPTS,
     FILE_SINGLE,
-    FILE_UNKNOWN_SEQ_ID,
     RaiseError,
     SEQ_ID_DUMMY,
     SEQ_ID_MATE_1,