diff --git a/htsinfer/get_library_type.py b/htsinfer/get_library_type.py
index 0edfc36..ca68c4e 100644
--- a/htsinfer/get_library_type.py
+++ b/htsinfer/get_library_type.py
@@ -113,7 +113,7 @@ def _evaluate_mate_relationship(
             ids_2: As `ids_1` but for the putative second mate file.
         """
         self.results.relationship = StatesTypeRelationship.not_mates
-        if ids_1 == ids_2:
+        if ids_1 == ids_2 and len(ids_1) > 0 and len(ids_2) > 0:
             if (
                 self.results.file_1 == StatesType.first_mate and
                 self.results.file_2 == StatesType.second_mate
@@ -136,7 +136,7 @@ def _evaluate_mate_relationship(
             self.mapping.evaluate()
             self._align_mates()
         else:
-            LOGGER.warning(
+            LOGGER.debug(
                 "Library source is not determined, "
                 "mate relationship cannot be inferred by alignment."
             )
@@ -181,7 +181,9 @@ def _align_mates(self):
                 seq_id2 = read2.query_name
                 if seq_id2 != previous_seq_id2 \
                         and previous_seq_id2 is not None:
-                    if self._compare_alignments(mate1[read_counter], reads2):
+                    if read_counter < len(mate1) and self._compare_alignments(
+                        mate1[read_counter], reads2
+                    ):
                         concordant += 1
                     reads2.clear()
                     read_counter += 1
@@ -189,9 +191,13 @@ def _align_mates(self):
                     reads2.append(read2)
             previous_seq_id2 = seq_id2
 
-        if self._compare_alignments(mate1[read_counter], reads2):
+        if read_counter < len(mate1) and self._compare_alignments(
+            mate1[read_counter], reads2
+        ):
             concordant += 1
-
+        LOGGER.debug(f"Number of mapped reads file 1: {len(mate1)}")
+        LOGGER.debug(f"Number of mapped reads file 2: {read_counter}")
+        LOGGER.debug(f"Number of concordant reads: {concordant}")
         self._update_relationship(concordant, read_counter)
 
         samfile1.close()
@@ -325,12 +331,14 @@ def evaluate(self) -> None:
 
                 if self.seq_id_format is None:
                     self.result = StatesType.not_available
-                    raise MetadataWarning(
+                    LOGGER.warning(
                         "Could not determine sequence identifier format."
                     )
-                LOGGER.debug(
-                    f"Sequence identifier format: {self.seq_id_format.name}"
-                )
+                else:
+                    LOGGER.debug(
+                        "Sequence identifier format: "
+                        f"{self.seq_id_format.name}"
+                    )
 
                 # Ensure that remaining records are compatible with sequence
                 # identifier format and library type determined from first
@@ -339,28 +347,33 @@ def evaluate(self) -> None:
                     "Checking consistency of remaining reads with initially "
                     "determined identifier format and library type..."
                 )
-                for record in seq_iter:
-                    records += 1
-                    try:
-                        self._get_read_type(
-                            seq_id=record[0],
-                            regex=self.seq_id_format.value,
-                        )
-                    except (
-                        InconsistentFastqIdentifiers,
-                        UnknownFastqIdentifier,
-                    ) as exc:
-                        self.result = StatesType.not_available
-                        raise MetadataWarning(
-                            f"{type(exc).__name__}: {str(exc)}"
-                        ) from exc
+                if self.seq_id_format is not None:
+                    for record in seq_iter:
+                        records += 1
+                        try:
+                            self._get_read_type(
+                                seq_id=record[0],
+                                regex=self.seq_id_format.value,
+                            )
+                        except (
+                            InconsistentFastqIdentifiers,
+                            UnknownFastqIdentifier,
+                        ) as exc:
+                            self.result = StatesType.not_available
+                            raise MetadataWarning(
+                                f"{type(exc).__name__}: {str(exc)}"
+                            ) from exc
+                    LOGGER.debug(f"Total records processed: {records}")
+                else:
+                    LOGGER.debug(
+                        "Could not determine sequence identifier format. "
+                        "Skipping consistency check for the remaining reads."
+                    )
 
         except (OSError, ValueError) as exc:
             self.result = StatesType.not_available
             raise FileProblem(f"{type(exc).__name__}: {str(exc)}") from exc
 
-        LOGGER.debug(f"Total records processed: {records}")
-
     def _get_read_type(
         self,
         seq_id: str,
diff --git a/htsinfer/get_read_orientation.py b/htsinfer/get_read_orientation.py
index 6e3c7a7..93b7c01 100644
--- a/htsinfer/get_read_orientation.py
+++ b/htsinfer/get_read_orientation.py
@@ -80,7 +80,7 @@ def evaluate(self) -> ResultsOrientation:
                      or self.library_source.file_2.short_name is not None):
             self.mapping.evaluate()
         else:
-            LOGGER.warning(
+            LOGGER.debug(
                 "Library source is not determined, "
                 "read orientation cannot be inferred by alignment."
             )
diff --git a/tests/test_get_library_type.py b/tests/test_get_library_type.py
index 738840a..9682bf7 100644
--- a/tests/test_get_library_type.py
+++ b/tests/test_get_library_type.py
@@ -214,12 +214,6 @@ def test_evaluate_single(self):
         test_instance.evaluate()
         assert test_instance.result == StatesType.single
 
-    def test_evaluate_unknown_seq_id(self):
-        """Evaluate file with identifiers of an unknown format."""
-        test_instance = GetFastqType(path=FILE_UNKNOWN_SEQ_ID)
-        with pytest.raises(MetadataWarning):
-            test_instance.evaluate()
-
     def test_evaluate_inconsistent_identifiers_single_mate(self):
         """Raise ``MetadataWarning`` by passing a file with inconsistent
         identifiers, suggesting a single-end library first, then a paired-end