diff --git a/errors.py b/errors.py
index f9c81aed..4d369652 100644
--- a/errors.py
+++ b/errors.py
@@ -4,4 +4,8 @@ class QCError(RuntimeError):
     '''Indicates a failure at a QC step.'''
     
     def __init__(self, reason):
-        super(QCError, self).__init__(reason)
\ No newline at end of file
+        super(QCError, self).__init__(reason)
+
+class InvalidBamHeaderError(ValueError):
+    '''Indicates a malformed or otherwise unusable bam file header'''
+    pass
\ No newline at end of file
diff --git a/read_utils.py b/read_utils.py
index 7fc3cd86..5d5d2f19 100755
--- a/read_utils.py
+++ b/read_utils.py
@@ -1388,6 +1388,9 @@ def minimap2_idxstats(inBam, refFasta, outBam=None, outStats=None,
     ref_indexed = util.file.mkstempfname('.reference.fasta')
     shutil.copyfile(refFasta, ref_indexed)
 
+    if samtools.isEmpty(inBam):
+        log.warning("The input bam file appears to have zero reads: %s", inBam)
+
     mm2.align_bam(inBam, ref_indexed, bam_aligned)
     
     if filterReadsAfterAlignment:
@@ -1401,6 +1404,10 @@ def minimap2_idxstats(inBam, refFasta, outBam=None, outStats=None,
         shutil.move(bam_aligned, bam_filtered)
 
     if outStats is not None:
+        # index the final bam before calling idxstats
+        # but only if it is a bam or cram file (sam cannot be indexed)
+        if (bam_filtered.endswith(".bam") or bam_filtered.endswith(".cram")):
+            samtools.index(bam_filtered)
         samtools.idxstats(bam_filtered, outStats)
 
     if outBam is None:
diff --git a/tools/bwa.py b/tools/bwa.py
index 09a9d180..799ace8d 100644
--- a/tools/bwa.py
+++ b/tools/bwa.py
@@ -16,6 +16,7 @@
 import tools.picard
 import util.file
 import util.misc
+from errors import *
 
 TOOL_NAME = 'bwa'
 
diff --git a/tools/minimap2.py b/tools/minimap2.py
index 0162caa3..e20f0e63 100644
--- a/tools/minimap2.py
+++ b/tools/minimap2.py
@@ -14,6 +14,7 @@
 import tools.picard
 import util.file
 import util.misc
+from errors import *
 
 TOOL_NAME = 'minimap2'
 
@@ -80,6 +81,7 @@ def align_bam(self, inBam, refDb, outBam, options=None,
                     log.warning("No alignment output for RG %s in file %s against %s", rg, inBam, refDb)
 
             if len(align_bams)==0:
+                log.warning("All read groups in file %s appear to be empty.", inBam)
                 with util.file.tempfname('.empty.sam') as empty_sam:
                     samtools.dumpHeader(inBam, empty_sam)
                     samtools.sort(empty_sam, outBam)
@@ -92,6 +94,7 @@ def align_bam(self, inBam, refDb, outBam, options=None,
                     picardOptions=picardOptions,
                     JVMmemory=JVMmemory
                 )
+
                 for bam in align_bams:
                     os.unlink(bam)
 
@@ -181,8 +184,10 @@ def align_one_rg(self, inBam, refDb, outBam, rgid=None, preset=None, options=Non
 
         # perform actual alignment
         if samtools.isEmpty(one_rg_inBam):
+            log.warning("Input file %s appears to lack reads for RG '%s'", inBam, rgid)
             # minimap doesn't like empty inputs, so copy empty bam through
-            samtools.sort(one_rg_inBam, outBam)
+            # samtools.sort(one_rg_inBam, outBam)
+            self.align_cmd(one_rg_inBam, refDb, outBam, options=options, threads=threads)
         else:
             self.align_cmd(one_rg_inBam, refDb, outBam, options=options, threads=threads)
 
diff --git a/tools/novoalign.py b/tools/novoalign.py
index 60e91571..80d7ec9e 100644
--- a/tools/novoalign.py
+++ b/tools/novoalign.py
@@ -7,12 +7,6 @@
     either in $PATH or $NOVOALIGN_PATH.
 '''
 
-import tools
-import tools.picard
-import tools.samtools
-import util.file
-import util.misc
-
 import logging
 import os
 import os.path
@@ -21,11 +15,16 @@
 import stat
 import sys
 
+import tools
+import tools.picard
+import tools.samtools
+import util.file
+import util.misc
+from errors import *
+
 _log = logging.getLogger(__name__)
 
 TOOL_NAME = "novoalign"
-#TOOL_VERSION = "3.09.04"
-
 
 class NovoalignTool(tools.Tool):
 
@@ -217,7 +216,3 @@ def index_fasta(self, refFasta, k=None, s=None):
             os.chmod(outfname, mode)
         except (IOError, OSError):
             _log.warning('could not chmod "%s", this is likely OK', refFasta)
-
-
-class InvalidBamHeaderError(ValueError):
-    pass