diff --git a/_pytest/debugging.py b/_pytest/debugging.py
index 23d94e688..aa6311013 100644
--- a/_pytest/debugging.py
+++ b/_pytest/debugging.py
@@ -2,6 +2,7 @@
 from __future__ import absolute_import, division, print_function
 import pdb
 import sys
+from doctest import UnexpectedException
 
 
 def pytest_addoption(parser):
@@ -95,10 +96,9 @@ def _enter_pdb(node, excinfo, rep):
 
 
 def _postmortem_traceback(excinfo):
-    # A doctest.UnexpectedException is not useful for post_mortem.
-    # Use the underlying exception instead:
-    from doctest import UnexpectedException
     if isinstance(excinfo.value, UnexpectedException):
+        # A doctest.UnexpectedException is not useful for post_mortem.
+        # Use the underlying exception instead:
         return excinfo.value.exc_info[2]
     else:
         return excinfo._excinfo[2]
diff --git a/changelog/1810.bugfix b/changelog/1810.bugfix
new file mode 100644
index 000000000..c91ed47d0
--- /dev/null
+++ b/changelog/1810.bugfix
@@ -0,0 +1 @@
+Move import of ``doctest.UnexpectedException`` to top-level to avoid possible errors when using ``--pdb``.