diff --git a/crossplane/errors.py b/crossplane/errors.py
index bcb7904..cd1d616 100644
--- a/crossplane/errors.py
+++ b/crossplane/errors.py
@@ -10,9 +10,9 @@ def __init__(self, strerror, filename, lineno):
 
     def __str__(self):
         if self.lineno is not None:
-            return '%s in %s:%s' % self.args
+            return '%s in %s:%s' % (self.strerror, self.filename, self.lineno)
         else:
-            return '%s in %s' % self.args
+            return '%s in %s' % (self.strerror, self.filename)
 
 
 class NgxParserSyntaxError(NgxParserBaseException):
diff --git a/tests/test_analyze.py b/tests/test_analyze.py
index e407ef4..fc0fdea 100644
--- a/tests/test_analyze.py
+++ b/tests/test_analyze.py
@@ -1,5 +1,6 @@
 # -*- coding: utf-8 -*-
 import crossplane
+from crossplane.errors import NgxParserBaseException, NgxParserSyntaxError
 
 
 def test_state_directive():
@@ -57,3 +58,21 @@ def test_flag_directive_args():
             raise Exception('bad args for flag directive: ' + repr(args))
         except crossplane.errors.NgxParserDirectiveArgumentsError as e:
             assert e.strerror.endswith('it must be "on" or "off"')
+
+
+def test_exception_str_with_lineno():
+    # str() with a line number should include all three fields
+    e = NgxParserBaseException('some error', 'nginx.conf', 42)
+    assert str(e) == 'some error in nginx.conf:42'
+
+
+def test_exception_str_without_lineno():
+    # str() with lineno=None must not raise TypeError (#93)
+    e = NgxParserBaseException('some error', 'nginx.conf', None)
+    assert str(e) == 'some error in nginx.conf'
+
+
+def test_exception_str_subclass_without_lineno():
+    # same contract holds for subclasses
+    e = NgxParserSyntaxError('unexpected "}"', 'nginx.conf', None)
+    assert str(e) == 'unexpected "}" in nginx.conf'