diff --git a/cvs2svn_lib/checkout_internal.py b/cvs2svn_lib/checkout_internal.py
index 8fdc132c9..6ce437612 100644
--- a/cvs2svn_lib/checkout_internal.py
+++ b/cvs2svn_lib/checkout_internal.py
@@ -102,7 +102,10 @@
 
 
 class TextRecord(object):
-  """Bookkeeping data for the text of a single CVSRevision."""
+  """Bookkeeping data for the text of a single CVSRevision.
+
+  The text is stored as the list of logical lines manipulated
+  by RCSStream."""
 
   __slots__ = ['id', 'refcount']
 
@@ -134,15 +137,23 @@ def decrement_refcount(self, text_record_db):
     if self.refcount == 0:
       text_record_db.discard(self.id)
 
-  def checkout(self, text_record_db):
+  def checkout_as_lines(self, text_record_db):
     """Workhorse of the checkout process.
 
-    Return the text for this revision, decrement our reference count,
-    and update the databases depending on whether there will be future
-    checkouts."""
+    Return the text for this revision as a list of logical lines,
+    decrement our reference count, and update the databases depending
+    on whether there will be future checkouts."""
 
     raise NotImplementedError()
 
+  def checkout(self, text_record_db):
+    """Return the text for this revision.
+
+    Just as checkout_as_lines(), but returns the text as a flat text
+    string."""
+
+    return "".join(self.checkout_as_lines(text_record_db))
+
   def free(self, text_record_db):
     """This instance will never again be checked out; free it.
 
@@ -158,7 +169,7 @@ class FullTextRecord(TextRecord):
   These records are used for revisions whose fulltext was determined
   by the InternalRevisionCollector during FilterSymbolsPass.  The
   fulltext for such a revision is is stored in the delta_db as a
-  single string."""
+  list of logical lines manipulated by RCSStream."""
 
   __slots__ = []
 
@@ -168,10 +179,10 @@ def __getstate__(self):
   def __setstate__(self, state):
     (self.id, self.refcount,) = state
 
-  def checkout(self, text_record_db):
-    text = text_record_db.delta_db[self.id]
+  def checkout_as_lines(self, text_record_db):
+    lines = text_record_db.delta_db[self.id]
     self.decrement_refcount(text_record_db)
-    return text
+    return lines
 
   def free(self, text_record_db):
     del text_record_db.delta_db[self.id]
@@ -205,26 +216,26 @@ def __setstate__(self, state):
   def increment_dependency_refcounts(self, text_record_db):
     text_record_db[self.pred_id].refcount += 1
 
-  def checkout(self, text_record_db):
-    base_text = text_record_db[self.pred_id].checkout(text_record_db)
-    rcs_stream = RCSStream(base_text)
+  def checkout_as_lines(self, text_record_db):
+    base_lines = text_record_db[self.pred_id].checkout_as_lines(text_record_db)
+    rcs_stream = RCSStream(base_lines)
     delta_text = text_record_db.delta_db[self.id]
     rcs_stream.apply_diff(delta_text)
-    text = rcs_stream.get_text()
+    lines = rcs_stream.get_lines()
     del rcs_stream
     self.refcount -= 1
     if self.refcount == 0:
-      # This text will never be needed again; just delete ourselves
+      # This lines will never be needed again; just delete ourselves
       # without ever having stored the fulltext to the checkout
       # database:
       del text_record_db[self.id]
     else:
       # Store a new CheckedOutTextRecord in place of ourselves:
-      text_record_db.checkout_db['%x' % self.id] = text
+      text_record_db.checkout_db['%x' % self.id] = lines
       new_text_record = CheckedOutTextRecord(self.id)
       new_text_record.refcount = self.refcount
       text_record_db.replace(new_text_record)
-    return text
+    return lines
 
   def free(self, text_record_db):
     del text_record_db.delta_db[self.id]
@@ -241,7 +252,8 @@ class CheckedOutTextRecord(TextRecord):
 
   These records are used for revisions whose fulltext has been
   computed already during OutputPass.  The fulltext for such a
-  revision is stored in the text_record_db as a single string."""
+  revision is stored in the text_record_db as a list of logical
+  lines manipulated by RCSStream."""
 
   __slots__ = []
 
@@ -251,10 +263,10 @@ def __getstate__(self):
   def __setstate__(self, state):
     (self.id, self.refcount,) = state
 
-  def checkout(self, text_record_db):
-    text = text_record_db.checkout_db['%x' % self.id]
+  def checkout_as_lines(self, text_record_db):
+    lines = text_record_db.checkout_db['%x' % self.id]
     self.decrement_refcount(text_record_db)
-    return text
+    return lines
 
   def free(self, text_record_db):
     del text_record_db.checkout_db['%x' % self.id]
@@ -533,7 +545,7 @@ def set_revision_info(self, revision, log, text):
         # This is revision 1.1.  Write its fulltext:
         text_record = FullTextRecord(cvs_rev_id)
         self.revision_collector._writeout(
-            text_record, self._rcs_stream.get_text()
+            text_record, self._rcs_stream.get_lines()
             )
 
         # There will be no more trunk revisions delivered, so free the
diff --git a/cvs2svn_lib/rcs_stream.py b/cvs2svn_lib/rcs_stream.py
index 0963956dc..c4ad79a07 100644
--- a/cvs2svn_lib/rcs_stream.py
+++ b/cvs2svn_lib/rcs_stream.py
@@ -174,13 +174,21 @@ class RCSStream:
   def __init__(self, text):
     """Instantiate and initialize the file content with TEXT."""
 
-    self.set_text(text)
+    if isinstance(text, bytes):
+      self.set_text(text)
+    else:
+      self.set_lines(text)
 
   def get_text(self):
     """Return the current file content."""
 
     return "".join(self._lines)
 
+  def get_lines(self):
+    """Return the current file content as list of logical lines."""
+
+    return self._lines
+
   def set_lines(self, lines):
     """Set the current contents to the specified LINES.
 
diff --git a/run-tests.py b/run-tests.py
index 7eaee0858..dd7594d85 100755
--- a/run-tests.py
+++ b/run-tests.py
@@ -3345,6 +3345,40 @@ def internal_co_keywords():
     raise Failure()
 
 
+@Cvs2SvnTestFunction
+def internal_co_broken_rcsfile():
+  "check if internal co can handle broken delta"
+
+  # This is a test for GitHub issue 18.
+
+  # Check if internal co can handle broken delta which inserts unterminated
+  # line other than after last line.  Although it does not seem such a
+  # "broken" RCS file could be generated by normal RCS/CVS operation,
+  # RCS/CVS can handle it.
+
+  ### TODO: Once an unterminated line is inserted in a middle of lines
+  ### in line buffer, there can be more critical situations than one
+  ### that the irregular.txt,v has. So we need to add such test data.
+
+  rcs_conv = ensure_conversion(
+      'gh-issue-18-irregular-rcsfile',
+      args=['--use-rcs', '--default-eol=native'],
+      dumpfile='broken-rcsfile-rcs.dump',
+      )
+  conv = ensure_conversion(
+      'gh-issue-18-irregular-rcsfile',
+      args=['--default-eol=native'],
+      dumpfile='broken-rcsfile-int.dump',
+      )
+  if conv.output_found(r'WARNING\: internal problem\: leftover revisions'):
+    raise Failure()
+  rcs_lines = list(open(rcs_conv.dumpfile, 'rb'))
+  lines = list(open(conv.dumpfile, 'rb'))
+  # Compare all lines following the repository UUID:
+  if lines[3:] != rcs_lines[3:]:
+    raise Failure()
+
+
 @Cvs2SvnTestFunction
 def timestamp_chaos():
   "test timestamp adjustments"
@@ -4237,10 +4271,11 @@ def vendor_1_1_not_root():
     internal_co_exclude,
     internal_co_trunk_only,
     internal_co_keywords,
+    internal_co_broken_rcsfile,
     leftover_revs,
     requires_internal_co,
-    timestamp_chaos,
 # 140:
+    timestamp_chaos,
     symlinks,
     empty_trunk_path,
     preferred_parent_cycle,
@@ -4250,8 +4285,8 @@ def vendor_1_1_not_root():
     add_on_branch,
     main_git,
     main_git2,
-    main_git_merged,
 # 150:
+    main_git_merged,
     main_git2_merged,
     git_options,
     main_hg,
@@ -4261,8 +4296,8 @@ def vendor_1_1_not_root():
     EOLVariants('LF'),
     EOLVariants('CR'),
     EOLVariants('CRLF'),
-    EOLVariants('native'),
 # 160:
+    EOLVariants('native'),
     no_revs_file,
     mirror_keyerror_test,
     exclude_ntdb_test,
@@ -4272,8 +4307,8 @@ def vendor_1_1_not_root():
     missing_deltatext,
     transform_unlabeled_branch_name,
     ignore_unlabeled_branch,
-    exclude_unlabeled_branch,
 # 170:
+    exclude_unlabeled_branch,
     unlabeled_branch_name_collision,
     collision_with_unlabeled_branch_name,
     many_deletes,
@@ -4283,8 +4318,8 @@ def vendor_1_1_not_root():
     exclude_symbol_default,
     add_on_branch2,
     branch_from_vendor_branch,
-    strange_default_branch,
 # 180:
+    strange_default_branch,
     move_parent,
     log_message_eols,
     missing_vendor_branch,
diff --git a/test-data/gh-issue-18-irregular-rcsfile-cvsrepos/irregular.txt,v b/test-data/gh-issue-18-irregular-rcsfile-cvsrepos/irregular.txt,v
new file mode 100644
index 000000000..c6a7b2471
--- /dev/null
+++ b/test-data/gh-issue-18-irregular-rcsfile-cvsrepos/irregular.txt,v
@@ -0,0 +1,122 @@
+head	1.6;
+access;
+symbols;
+locks; strict;
+comment	@# @;
+
+
+1.6
+date	2021.12.21.11.21.57;	author futatuki;	state Exp;
+branches;
+next	1.5;
+
+1.5
+date	2021.11.26.04.16.45;	author futatuki;	state Exp;
+branches;
+next	1.4;
+
+1.4
+date	2021.11.26.04.14.23;	author futatuki;	state Exp;
+branches;
+next	1.3;
+
+1.3
+date	2021.11.26.03.46.06;	author futatuki;	state Exp;
+branches;
+next	1.2;
+
+1.2
+date	2021.11.26.03.41.39;	author futatuki;	state Exp;
+branches
+	1.2.1.1;
+next	1.1;
+
+1.1
+date	2021.11.26.03.34.28;	author futatuki;	state Exp;
+branches;
+next	;
+
+1.2.1.1
+date	2021.12.21.11.44.31;	author futatuki;	state Exp;
+branches;
+next	;
+
+
+desc
+@create a new file irregular.txt
+@
+
+
+1.6
+log
+@r1.6:
+@
+text
+@aaa
+bbb
+bc
+cc
+hhh@
+
+
+1.5
+log
+@r1.5:
+@
+text
+@d3 1
+a3 1
+c@
+
+
+1.4
+log
+@r1.4:
+@
+text
+@d5 1
+a5 1
+ggg@
+
+
+1.3
+log
+@r1.3: s/eee/fff/
+@
+text
+@d4 1
+a4 1
+fff@
+
+
+1.2
+log
+@r1.2: s/ddd/eee/
+@
+text
+@d4 1
+a4 1
+eee@
+
+
+1.2.1.1
+log
+@rev:1.2.1.1
+@
+text
+@d4 1
+a4 2
+ccc
+eeeggg
+@
+
+
+1.1
+log
+@r1.1
+@
+text
+@d4 1
+a4 1
+ddd@
+