Raise error when main document part does not exist

2 files changed, 19 insertions, 2 deletions

diff --git a/mammoth/docx/__init__.py b/mammoth/docx/__init__.py
index 8ecf8c9..b155cff 100644
--- a/mammoth/docx/__init__.py
+++ b/mammoth/docx/__init__.py
@@ -77,7 +77,7 @@ def _find_document_filename(zip_file, relationships):
     ] + ["word/document.xml"]
     valid_targets = list(filter(lambda target: zip_file.exists(target), targets))
     if len(valid_targets) == 0:
-        return None
+        raise IOError("Could not find main document part. Are you sure this is a valid .docx file?")
     else:
         return valid_targets[0]
 
diff --git a/tests/docx/docx_tests.py b/tests/docx/docx_tests.py
index 8bd5b36..d450424 100644
--- a/tests/docx/docx_tests.py
+++ b/tests/docx/docx_tests.py
@@ -5,7 +5,7 @@ import zipfile
 from nose.tools import istest, assert_equal
 
 from mammoth import docx, documents
-from ..testing import test_path
+from ..testing import assert_raises, test_path
 
 
 @istest
@@ -62,6 +62,23 @@ def main_document_is_found_using_package_relationships():
     assert_equal(expected_document, result.value)
 
 
+@istest
+def error_is_raised_when_main_document_part_does_not_exist():
+    fileobj = _create_zip({
+        "_rels/.rels": textwrap.dedent("""\
+            <?xml version="1.0" encoding="utf-8"?>
+            <Relationships xmlns="http://schemas.openxmlformats.org/package/2006/relationships">
+                <Relationship Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/officeDocument" Target="/word/document2.xml" Id="rId1"/>
+            </Relationships>
+        """),
+    })
+    error = assert_raises(IOError, lambda: docx.read(fileobj=fileobj))
+    assert_equal(
+        "Could not find main document part. Are you sure this is a valid .docx file?",
+        str(error),
+    )
+
+
 def _create_zip(files):
     fileobj = io.BytesIO()