diff --git a/prost/excel.py b/prost/excel.py
index 9aea505..2b08b5b 100644
--- a/prost/excel.py
+++ b/prost/excel.py
@@ -441,6 +441,7 @@ def create_one(in_prefix, out_prefix, tsv_files):
         perform_formatting(sheet, header, num_rows, fmt_percent,
                 FMT_PERCENT_CONDITIONAL_TWO_COLOR,
                 FMT_PERCENT_CONDITIONAL_THREE_COLOR)
+    book.close()
     return book_filename
 
 def create_many(in_prefix, out_prefix, tsv_files):
@@ -465,6 +466,7 @@ def create_many(in_prefix, out_prefix, tsv_files):
         perform_formatting(sheet, header, num_rows, fmt_percent,
                 FMT_PERCENT_CONDITIONAL_TWO_COLOR,
                 FMT_PERCENT_CONDITIONAL_THREE_COLOR)
+    book.close()
     return book_filenames
 
 
diff --git a/setup.py b/setup.py
index 05b66f1..4d70931 100644
--- a/setup.py
+++ b/setup.py
@@ -47,7 +47,7 @@
     # project is installed. For an analysis of "install_requires" vs pip's
     # requirements files see:
     # https://packaging.python.org/en/latest/technical.html#install-requires-vs-requirements-files
-    install_requires=['enum34<=1.0.4', 'docopt', 'xlsxwriter', 'psutil'],
+    install_requires=['enum34<=1.0.4', 'docopt==0.6.2', 'xlsxwriter>=1.1.4,<=1.1.7', 'psutil<=5.6.1'],
 
     # List additional groups of dependencies here (e.g. development dependencies).
     # You can install these using the following syntax, for example: